code: def set_backup_heartbeat(self, interface_id):
          
          self.interface.set_unset(interface_id, 'backup_heartbeat')
          self._engine.update()
docs: Set this interface as the backup heartbeat interface.
          Clusters and Master NGFW Engines only.
          
          :param str,int interface_id: interface as backup
          :raises InterfaceNotFound: specified interface is not found
          :raises UpdateElementFailed: failure to update interface
          :return: None