Datasource for retrieving GCS service account (hashicorp#1110)

* Datasource for retrieving GCS service account

* Removing duplicated argument

* Gofmt post resolving conflicts

* Addressing review comment

docs/d/google_storage_project_service_account.html.markdown

@@ -0,0 +1,36 @@
+---
+layout: "google"
+page_title: "Google: google_storage_project_service_account"
+sidebar_current: "docs-google-datasource-storage-project-service-account"
+description: |-
+  Get the email address of the project's Google Cloud Storage service account
+---
+
+# google\_storage\_project\_service\_account
+
+Use this data source to get the email address of the project's Google Cloud Storage service account.
+ For more information see 
+[API](https://cloud.google.com/storage/docs/json_api/v1/projects/serviceAccount).
+
+## Example Usage
+
+```hcl
+data "google_storage_project_service_account" "gcs_account" {}
+
+resource "google_pubsub_topic_iam_binding" "binding" {
+	topic       = "${google_pubsub_topic.topic.name}"
+	role        = "roles/pubsub.publisher"
+		  
+	members     = ["${data.google_storage_project_service_account.gcs_account.id}"]
+}
+```
+
+## Argument Reference
+
+There are no arguments available for this data source.
+
+## Attributes Reference
+
+The following attributes are exported:
+
+* `id` - The ID of the service account, which is its email address

google.erb

@@ -85,6 +85,9 @@
       <li<%= sidebar_current("docs-google-datasource-signed_url") %>>
         <a href="/docs/providers/google/d/signed_url.html">google_storage_object_signed_url</a>
       </li>
+      <li<%= sidebar_current("docs-google-datasource-storage-project-service-account") %>>
+        <a href="/docs/providers/google/d/google_storage_project_service_account.html">google_storage_project_service_account</a>
+      </li>
     </ul>
     </li>