_validatePayload = event => {
  let payload;
  try {
    payload = JSON.parse(Buffer.from(event.data, 'base64').toString());
  } catch (err) {
    throw new Error('Invalid Pub/Sub message: ' + err);
  }
  if (!payload.zone) {
    throw new Error("Attribute 'zone' missing from payload");
  } else if (!payload.label) {
    throw new Error("Attribute 'label' missing from payload");
  }
  return payload;
}
Validates that a request payload contains the expected fields.

@param {!object} payload the request payload to validate.
@return {!object} the payload object.