Comment 0 for bug 1831181

[Environment]

Xenial/Queens

[Description]

If no user_domain_id is provided then, the following exception will be raised by the
aodh-notifier component:

$ openstack alarm-history show 1424b6fb-1d9d-4d69-b00a-ad455aad0ecb

==> /var/log/aodh/aodh-notifier.log <==
2019-05-31 02:07:26.663 15274 ERROR aodh.notifier File "/usr/lib/python2.7/dist-packages/keystoneauth1/identity/generic/base.py", line 201, in get_auth_ref
2019-05-31 02:07:26.663 15274 ERROR aodh.notifier return self._plugin.get_auth_ref(session, **kwargs)
2019-05-31 02:07:26.663 15274 ERROR aodh.notifier File "/usr/lib/python2.7/dist-packages/keystoneauth1/identity/v2.py", line 63, in get_auth_ref
2019-05-31 02:07:26.663 15274 ERROR aodh.notifier authenticated=False, log=False)
2019-05-31 02:07:26.663 15274 ERROR aodh.notifier File "/usr/lib/python2.7/dist-packages/keystoneauth1/session.py", line 848, in post
2019-05-31 02:07:26.663 15274 ERROR aodh.notifier return self.request(url, 'POST', **kwargs)
2019-05-31 02:07:26.663 15274 ERROR aodh.notifier File "/usr/lib/python2.7/dist-packages/keystoneauth1/session.py", line 737, in request
2019-05-31 02:07:26.663 15274 ERROR aodh.notifier raise exceptions.from_response(resp, method, url)
2019-05-31 02:07:26.663 15274 ERROR aodh.notifier NotFound: (http://10.5.0.36:35357/v2.0/tokens): The resource could not be found. (HTTP 404) (Request-ID: req-50d54903-d2af-4fd1-be68-a92498d91b85)
2019-05-31 02:07:26.663 15274 ERROR aodh.notifier

Domain configuration for the aodh service is as follows:

root@juju-95497c-controller-1:/home/ubuntu# grep domain /etc/aodh/aodh.conf
gnocchi_external_domain_name = service_domain
project_domain_name = service_domain
user_domain_name = service_domain

### Explanation

The following line https://github.com/openstack/aodh/blob/master/aodh/keystone_client.py#L51
enforces the existence of the user_domain_id for getting the authenticated keystone session.

[Proposed Solution]

* Use user_domain_name as a fallback if no user_domain_id is provided.
* The charm templates should provide only user_domain_id if provided orelse
fallback to user_domain_name.