code: def validate_rmq_cluster_running_nodes(self, sentry_units):
          
          host_names = self.get_unit_hostnames(sentry_units)
          errors = []
  
          # Query every unit for cluster_status running nodes
          for query_unit in sentry_units:
              query_unit_name = query_unit.info['unit_name']
              running_nodes = self.get_rmq_cluster_running_nodes(query_unit)
  
              # Confirm that every unit is represented in the queried unit's
              # cluster_status running nodes output.
              for validate_unit in sentry_units:
                  val_host_name = host_names[validate_unit.info['unit_name']]
                  val_node_name = 'rabbit@{}'.format(val_host_name)
  
                  if val_node_name not in running_nodes:
                      errors.append('Cluster member check failed on {}: {} not '
                                    'in {}\n'.format(query_unit_name,
                                                     val_node_name,
                                                     running_nodes))
          if errors:
              return ''.join(errors)
docs: Check that all rmq unit hostnames are represented in the
          cluster_status output of all units.
  
          :param host_names: dict of juju unit names to host names
          :param units: list of sentry unit pointers (all rmq units)
          :returns: None if successful, otherwise return error message