Make the google unique ID of zone/project/id.

def MakeGoogleUniqueID(cloud_instance):
  """Make the google unique ID of zone/project/id."""
  if not (cloud_instance.zone and cloud_instance.project_id and
          cloud_instance.instance_id):
    raise ValueError("Bad zone/project_id/id: '%s/%s/%s'" %
                     (cloud_instance.zone, cloud_instance.project_id,
                      cloud_instance.instance_id))
  return "/".join([
      cloud_instance.zone.split("/")[-1], cloud_instance.project_id,
      cloud_instance.instance_id
  ])