'Check if a user is a member of a group.
:param str user_id: User ID.
:param str group_id: Group ID.
:raises keystone.exception.NotFound: If the user is not a member of the
group.
:raises keystone.exception.UserNotFound: If the user doesn\'t exist.
:raises keystone.exception.GroupNotFound: If the group doesn\'t exist.'

@abc.abstractmethod
def check_user_in_group(self, user_id, group_id):

    raise exception.NotImplemented()
