code: def set_primary_heartbeat(self, interface_id):
          
          self.interface.set_unset(interface_id, 'primary_heartbeat')
          self._engine.update()
docs: Set this interface as the primary heartbeat for this engine. 
          This will 'unset' the current primary heartbeat and move to
          specified interface_id.
          Clusters and Master NGFW Engines only.
          
          :param str,int interface_id: interface specified for primary mgmt
          :raises InterfaceNotFound: specified interface is not found
          :raises UpdateElementFailed: failed modifying interfaces
          :return: None