Trove-taskamager raises error when nova_compute_url isn't set

[Mahito OGURA]

I got the error log as follows in trove-taskmanager.

```
2016-08-24 06:57:02.080 ERROR oslo_service.periodic_task [-] Error during Manager.publish_quota_notifications
2016-08-24 06:57:02.080 TRACE oslo_service.periodic_task Traceback (most recent call last):
2016-08-24 06:57:02.080 TRACE oslo_service.periodic_task File "/usr/local/lib/python2.7/dist-packages/oslo_service/periodic_task.py", line 220, in run_periodic_tasks
2016-08-24 06:57:02.080 TRACE oslo_service.periodic_task task(self, context)
2016-08-24 06:57:02.080 TRACE oslo_service.periodic_task File "/opt/stack/trove/trove/taskmanager/manager.py", line 410, in publish_quota_notifications
2016-08-24 06:57:02.080 TRACE oslo_service.periodic_task nova_client = remote.create_nova_client(self.admin_context)
2016-08-24 06:57:02.080 TRACE oslo_service.periodic_task File "/opt/stack/trove/trove/common/remote.py", line 99, in nova_client
2016-08-24 06:57:02.080 TRACE oslo_service.periodic_task endpoint_type=CONF.nova_compute_endpoint_type)
2016-08-24 06:57:02.080 TRACE oslo_service.periodic_task File "/opt/stack/trove/trove/common/remote.py", line 58, in get_endpoint
2016-08-24 06:57:02.080 TRACE oslo_service.periodic_task raise exception.EmptyCatalog()
2016-08-24 06:57:02.080 TRACE oslo_service.periodic_task EmptyCatalog: Empty catalog.
2016-08-24 06:57:02.080 TRACE oslo_service.periodic_task
```

This error happens when nova_compute_url isn't set.
I looked source code and found the reason.

trove-taskmanager dosen't set `service_catalog`, when `admin_context` is set.[1]
When `nova_compute_url` isn't set, `nova_client` calls `get_endpoint`.[2]
When there isn't set `service_catalog`, `get_endpoint` raises `exception.EmptyCatalog()`.[3]
There aren't test cases, because`TestCreateNovaClient` already sets `service_catalog`.[4]

[1]: https://github.com/openstack/trove/blob/master/trove/taskmanager/manager.py#L49-L52
[2]: https://github.com/openstack/trove/blob/master/trove/common/remote.py#L90-L103
[3]: https://github.com/openstack/trove/blob/master/trove/common/remote.py#L57-L58
[4]: https://github.com/openstack/trove/blob/master/trove/tests/unittests/common/test_remote.py#L339-L362

I don't have a good fix idea.
please anyone fix this issue.

best regards

[Liu Yang (ly798)]

Looks similar to https://bugs.launchpad.net/trove/+bug/1402055