'Start/Stop host maintenance window.
On start, it triggers the migration of all instances to other hosts.
Consider the combination with :func:`set_host_enabled`.
:param str host:
The name of the host whose maintenance mode should be changed.
:param bool mode:
If `True`, go into maintenance mode. If `False`, leave the
maintenance mode.
:return: "on_maintenance" if switched to maintenance mode or
"off_maintenance" if maintenance mode got left.
:rtype: str'

def host_maintenance_mode(self, host, mode):

    raise NotImplementedError()
