'List groups a user is in.
:param str user_id: the user in question
:param hints: filter hints which the driver should
implement if at all possible.
:type hints: keystone.common.driver_hints.Hints
:returns: a list of group_refs or an empty list. See group schema in
:class:`~.IdentityDriverBase`.
:raises keystone.exception.UserNotFound: If the user doesn\'t exist.'

@abc.abstractmethod
def list_groups_for_user(self, user_id, hints):

    raise exception.NotImplemented()
