cinder db not updated when create cgsnapshot

[wangzhizheng]

when create cgsnapshot, the status of cgsnapshot will be stopped at "creating" not become "available".
error messages shows below in cinder/volume.log
2017-04-20 21:00:19.480 70467 ERROR oslo_messaging.rpc.dispatcher setattr(self, k, v)
2017-04-20 21:00:19.480 70467 ERROR oslo_messaging.rpc.dispatcher AttributeError: can't set attribute