'Updates a group.
Because the backend has no concept of volume grouping, cinder will
maintain all volume/group relationships. Because of this
functionality, there is no need to make any client calls; instead
simply returning out of this function allows cinder to properly
add/remove volumes from the group.'

@cinder_utils.trace
def update_group(self, context, group, add_volumes=None, remove_volumes=None):

    LOG.debug('Updating    group.')
    if (not utils.is_group_a_cg_snapshot_type(group)):
        raise NotImplementedError()
    return (None, None, None)
