revert-to-snapshot operation failed if the cinder volume has a metadata.

Issue

• revert-to-snapshot operation failed if the cinder volume has a metadata.
  cinder-volume.log will have logs like below.

DEBUG cinder.manager [req-26d544bf-10f1-4748-8589-6c7deded0c99 - - - - -] Notifying Schedulers of capabilities ... _publish_service_capabilities /usr/lib
/python2.7/site-packages/cinder/manager.py:194
INFO cinder.volume.manager [req-e1ca2ca3-81da-48fb-952e-3d3484e221af a219576c0c7a431e8fe54f4b78011d7e 2e0b0df6eff14a12b715b1459f9fd731 - default default]
 Start to perform revert to snapshot process.
ERROR root [req-e1ca2ca3-81da-48fb-952e-3d3484e221af a219576c0c7a431e8fe54f4b78011d7e 2e0b0df6eff14a12b715b1459f9fd731 - default default] Original exception being dropped: ['Traceback (most recent call last):\n', '  File "/usr/lib/python2.7/site-packages/cinder/volume/manager.py", line 998, in revert_to_snapshot\n    "revert.start")\n', '  File "/usr/lib/python2.7/site-packages/cinder/volume/manager.py", line 2584, in _notify_about_snapshot_usage\n    extra_usage_info=extra_usage_info, host=self.host)\n', '  File "/usr/lib/python2.7/site-packages/cinder/utils.py", line 1197, in wrapped\n    return f(*args, **kwargs)\n', '  File "/usr/lib/python2.7/site-packages/cinder/volume/utils.py"
, line 197, in notify_about_snapshot_usage\n    usage_info = _usage_from_snapshot(snapshot, context, **extra_usage_info)\n', '  File "/usr/lib/python2.7/site-packages/cinder/volume/utils.py", line 181, in _usage_from_snapshot\n    \'metadata\': null_safe_str(snapshot.metadata),\n', '  File "/usr/lib/python2.7/site-packages/oslo_versionedobjects/base.py", line 68, in getter\n    return getattr(self, attrname)\n', "AttributeError: 'Snapshot' object has no attribute '_obj_metadata'\n"]
ERROR oslo_messaging.rpc.server [req-e1ca2ca3-81da-48fb-952e-3d3484e221af a219576c0c7a431e8fe54f4b78011d7e 2e0b0df6eff14a12b715b1459f9fd731 - default default] Exception during message handling: AttributeError: 'Snapshot' object has no attribute '_obj_metadata'
ERROR oslo_messaging.rpc.server Traceback (most recent call last):
ERROR oslo_messaging.rpc.server   File "/usr/lib/python2.7/site-packages/oslo_messaging/rpc/server.py", line 163, in _process_incoming
ERROR oslo_messaging.rpc.server     res = self.dispatcher.dispatch(message)
ERROR oslo_messaging.rpc.server   File "/usr/lib/python2.7/site-packages/oslo_messaging/rpc/dispatcher.py", line 220, in dispatch
ERROR oslo_messaging.rpc.server     return self._do_dispatch(endpoint, method, ctxt, args)
ERROR oslo_messaging.rpc.server   File "/usr/lib/python2.7/site-packages/oslo_messaging/rpc/dispatcher.py", line 190, in _do_dispatch
ERROR oslo_messaging.rpc.server     result = func(ctxt, **new_args)
ERROR oslo_messaging.rpc.server   File "/usr/lib/python2.7/site-packages/cinder/volume/manager.py", line 1012, in revert_to_snapshot
ERROR oslo_messaging.rpc.server     "revert.end")
ERROR oslo_messaging.rpc.server   File "/usr/lib/python2.7/site-packages/cinder/volume/manager.py", line 2584, in _notify_about_snapshot_usage
ERROR oslo_messaging.rpc.server     extra_usage_info=extra_usage_info, host=self.host)
ERROR oslo_messaging.rpc.server   File "/usr/lib/python2.7/site-packages/cinder/utils.py", line 1197, in wrapped
ERROR oslo_messaging.rpc.server     return f(*args, **kwargs)
ERROR oslo_messaging.rpc.server   File "/usr/lib/python2.7/site-packages/cinder/volume/utils.py", line 197, in notify_about_snapshot_usage
ERROR oslo_messaging.rpc.server     usage_info = _usage_from_snapshot(snapshot, context, **extra_usage_info)
ERROR oslo_messaging.rpc.server   File "/usr/lib/python2.7/site-packages/cinder/volume/utils.py", line 181, in _usage_from_snapshot
ERROR oslo_messaging.rpc.server     'metadata': null_safe_str(snapshot.metadata),
ERROR oslo_messaging.rpc.server   File "/usr/lib/python2.7/site-packages/oslo_versionedobjects/base.py", line 68, in getter
ERROR oslo_messaging.rpc.server     return getattr(self, attrname)
ERROR oslo_messaging.rpc.server AttributeError: 'Snapshot' object has no attribute '_obj_metadata'
ERROR oslo_messaging.rpc.server 
DEBUG oslo_service.periodic_task [req-26d544bf-10f1-4748-8589-6c7deded0c99 - - - - -] Running periodic task VolumeManager.publish_service_capabilities run_periodic_tasks /usr/lib/python2.7/site-packages/oslo_service/periodic_task.py:215