Return all Google Storage scopes available on this VM.

def _GetGsScopes(self):
    """Return all Google Storage scopes available on this VM."""
    service_accounts = self.watcher.GetMetadata(metadata_key=self.metadata_key)
    try:
      scopes = service_accounts[self.service_account]['scopes']
      return list(GS_SCOPES.intersection(set(scopes))) if scopes else None
    except KeyError:
      return None