ZEPPELIN-3825. Allow custom service account for GCSNotebookRepo

* Added ZEPPELIN_NOTEBOOK_GCS_CREDENTIALS_FILE to ConfVars
* Added functionality in GCSNotebookRepo and OldGCSNotebookRepo
  to get credentials from CREDENTIALS_FILE
* Updated doc string and documentation

conf/zeppelin-site.xml.template

@@ -84,6 +84,15 @@
  </description>
 </property>
 
+<property>
+  <name>zeppelin.notebook.gcs.credentialsJsonFilePath</name>
+  <value>path/to/key.json</value>
+  <description>
+    Path to GCS credential key file for authentication with Google Storage.
+ </description>
+</property>
+
+
 <property>
   <name>zeppelin.notebook.storage</name>
   <value>org.apache.zeppelin.notebook.repo.GCSNotebookRepo</value>

docs/setup/storage/storage.md

@@ -317,7 +317,7 @@ Or, if you want to simultaneously use your local git storage with GCS, use the f
 ### Google Cloud API Authentication
 
 Note: On Google App Engine, Google Cloud Shell, and Google Compute Engine, these
-steps are not necessary, as build-in credentials are used by default.
+steps are not necessary if you are using the default built in service account.
 
 For more information, see [Application Default Credentials](https://cloud.google.com/docs/authentication/production)
 
@@ -351,11 +351,25 @@ for authentication with GCS, you will need a JSON service account key file.
    `/path/to/my/key.json`), and give it appropriate permissions. Ensure at
    least the user running the zeppelin daemon can read it.
 
-Then, point `GOOGLE_APPLICATION_CREDENTIALS` at your new key file in **zeppelin-env.sh**. For example:
+ If you wish to set this as your default credential file to access Google Services,
+ point `GOOGLE_APPLICATION_CREDENTIALS` at your new key file in **zeppelin-env.sh**. For example:
 
 ```bash
 export GOOGLE_APPLICATION_CREDENTIALS=/path/to/my/key.json
 ```
+If you do not want to use this key file as default credential file and want to specify a custom key
+file for authentication with GCS, update the following property :
+
+```xml
+<property>
+  <name>zeppelin.notebook.google.credentialsJsonFilePath</name>
+  <value>path/to/key.json</value>
+  <description>
+    Path to GCS credential key file for authentication with Google Storage.
+ </description>
+</property>
+```
+
 
 </br>
 ## Notebook Storage in ZeppelinHub  <a name="ZeppelinHub"></a>

zeppelin-interpreter/src/main/java/org/apache/zeppelin/conf/ZeppelinConfiguration.java

@@ -738,6 +738,7 @@ public class ZeppelinConfiguration extends XMLConfiguration {
     // whether homescreen notebook will be hidden from notebook list or not
     ZEPPELIN_NOTEBOOK_HOMESCREEN_HIDE("zeppelin.notebook.homescreen.hide", false),
     ZEPPELIN_NOTEBOOK_GCS_STORAGE_DIR("zeppelin.notebook.gcs.dir", ""),
+    ZEPPELIN_NOTEBOOK_GCS_CREDENTIALS_FILE("zeppelin.notebook.google.credentialsJsonFilePath", null),
     ZEPPELIN_NOTEBOOK_S3_BUCKET("zeppelin.notebook.s3.bucket", "zeppelin"),
     ZEPPELIN_NOTEBOOK_S3_ENDPOINT("zeppelin.notebook.s3.endpoint", "s3.amazonaws.com"),
     ZEPPELIN_NOTEBOOK_S3_TIMEOUT("zeppelin.notebook.s3.timeout", "120000"),

zeppelin-plugins/notebookrepo/gcs/src/main/java/org/apache/zeppelin/notebook/repo/GCSNotebookRepo.java

@@ -17,6 +17,8 @@
 
 package org.apache.zeppelin.notebook.repo;
 
+import com.google.auth.Credentials;
+import com.google.auth.oauth2.GoogleCredentials;
 import com.google.cloud.storage.Blob;
 import com.google.cloud.storage.BlobId;
 import com.google.cloud.storage.BlobInfo;
@@ -29,6 +31,8 @@ import com.google.common.base.Optional;
 import com.google.common.base.Preconditions;
 import com.google.common.base.Strings;
 import com.google.gson.JsonParseException;
+
+import java.io.FileInputStream;
 import java.io.IOException;
 import java.util.Arrays;
 import java.util.Collections;
@@ -54,7 +58,9 @@ import org.slf4j.LoggerFactory;
  * object store, so this "directory" should not itself be an object. Instead, it represents the base
  * path for the note.json files.
  *
- * Authentication is provided by google-auth-library-java.
+ * Authentication is provided by google-auth-library-java. A custom json key file path
+ * can be specified by zeppelin.notebook.google.credentialsJsonFilePath to connect with GCS
+ * If not specified the GOOGLE_APPLICATION_CREDENTIALS will be used to connect to GCS.
  * @see <a href="https://github.com/google/google-auth-library-java">
  *   google-auth-library-java</a>.
  */
@@ -113,7 +119,12 @@ public class GCSNotebookRepo implements NotebookRepo {
       this.notePathPattern = Pattern.compile("^(.+\\.zpln)$");
     }
 
-    this.storage = StorageOptions.getDefaultInstance().getService();
+    Credentials credentials = GoogleCredentials.getApplicationDefault();
+    String credentialJsonPath = zConf.getString(ConfVars.ZEPPELIN_NOTEBOOK_GCS_CREDENTIALS_FILE);
+    if (credentialJsonPath != null) {
+      credentials = GoogleCredentials.fromStream(new FileInputStream(credentialJsonPath));
+    }
+    this.storage = StorageOptions.newBuilder().setCredentials(credentials).build().getService();
   }
 
   private BlobId makeBlobId(String noteId, String notePath) throws IOException {

zeppelin-plugins/notebookrepo/gcs/src/main/java/org/apache/zeppelin/notebook/repo/OldGCSNotebookRepo.java

@@ -17,6 +17,8 @@
 
 package org.apache.zeppelin.notebook.repo;
 
+import com.google.auth.Credentials;
+import com.google.auth.oauth2.GoogleCredentials;
 import com.google.cloud.storage.Blob;
 import com.google.cloud.storage.BlobId;
 import com.google.cloud.storage.BlobInfo;
@@ -39,6 +41,7 @@ import org.apache.zeppelin.user.AuthenticationInfo;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 
+import java.io.FileInputStream;
 import java.io.IOException;
 import java.util.ArrayList;
 import java.util.Arrays;
@@ -56,7 +59,9 @@ import java.util.regex.Pattern;
  * object store, so this "directory" should not itself be an object. Instead, it represents the base
  * path for the note.json files.
  *
- * Authentication is provided by google-auth-library-java.
+ * Authentication is provided by google-auth-library-java. A custom json key file path
+ * can be specified by zeppelin.notebook.google.credentialsJsonFilePath to connect with GCS
+ * If not specified the GOOGLE_APPLICATION_CREDENTIALS will be used to connect to GCS.
  * @see <a href="https://github.com/google/google-auth-library-java">
  *   google-auth-library-java</a>.
  */
@@ -115,7 +120,13 @@ public class OldGCSNotebookRepo implements OldNotebookRepo {
       this.noteNamePattern = Pattern.compile("^([^/]+)/note\\.json$");
     }
 
-    this.storage = StorageOptions.getDefaultInstance().getService();
+
+    Credentials credentials = GoogleCredentials.getApplicationDefault();
+    String credentialJsonPath = zConf.getString(ConfVars.ZEPPELIN_NOTEBOOK_GCS_CREDENTIALS_FILE);
+    if (credentialJsonPath != null) {
+      credentials = GoogleCredentials.fromStream(new FileInputStream(credentialJsonPath));
+    }
+    this.storage = StorageOptions.newBuilder().setCredentials(credentials).build().getService();
   }
 
   private BlobId makeBlobId(String noteId) {