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

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

    add_volumes = (add_volumes if add_volumes else [])
    remove_volumes = (remove_volumes if remove_volumes else [])
    for vol in add_volumes:
        add_data = {'vol-id': vol['id'], 'cg-id': group['id']}
        self.client.req('consistency-group-volumes', 'POST', add_data, ver='v2')
    for vol in remove_volumes:
        remove_data = {'vol-id': vol['id'], 'cg-id': group['id']}
        self.client.req('consistency-group-volumes', 'DELETE', remove_data, name=group['id'], ver='v2')
    return (None, None, None)
