code: def find_mgmt_interface(self, mgmt):
          
          for intf in self:
              for allitf in intf.all_interfaces:
                  if isinstance(allitf, VlanInterface):
                      for vlan in allitf.interfaces:
                          if getattr(vlan, mgmt, None):
                              return allitf.interface_id
                  else:
                      if getattr(allitf, mgmt, None):
                          return intf.interface_id
docs: Find the management interface specified and return
          either the string representation of the interface_id.
          
          Valid options: primary_mgt, backup_mgt, primary_heartbeat,
              backup_heartbeat, outgoing, auth_request
          
          :return: str interface_id