backend/remote-state/gcloud: Use the context provided to configure().

Calling context.Background() from outside the main() function is
discouraged. The configure functions are only called from
"…/helper/schema".Backend.Configure which provides the Background context,
i.e. a long-living context we can use for backend communication.

backend/remote-state/gcloud/backend.go

@@ -58,11 +58,16 @@ func (b *Backend) configure(ctx context.Context) error {
 		return nil
 	}
 
-	data := schema.FromContextBackendConfig(ctx)
+	// ctx is a background context with the backend config added.
+	// Since no context is passed to RemoteClient.Get(), .Lock(), etc. but
+	// one is required for calling the GCP API, we're holding on to this
+	// context here and re-use it later.
+	b.storageContext = ctx
+
+	data := schema.FromContextBackendConfig(b.storageContext)
 
 	b.bucketName = data.Get("bucket").(string)
 	b.stateDir = data.Get("state_dir").(string)
-	b.storageContext = googleContext.Background()
 
 	var tokenSource oauth2.TokenSource