'Updates a group.
:param context: the context of the caller.
:param group: the group object.
:param add_volumes: a list of volume objects to be added.
:param remove_volumes: a list of volume objects to be removed.
:returns: model_update, add_volumes_update, remove_volumes_update'

def update_group(self, context, group, add_volumes=None, remove_volumes=None):

    return self.update_consistencygroup(context, group, add_volumes, remove_volumes)
