Create cluster proto buff message for API calls

def _to_pb(self):
        """ Create cluster proto buff message for API calls """
        client = self._instance._client
        location = client.instance_admin_client.location_path(
            client.project, self.location_id
        )
        cluster_pb = instance_pb2.Cluster(
            location=location,
            serve_nodes=self.serve_nodes,
            default_storage_type=self.default_storage_type,
        )
        return cluster_pb