Use get_notification_url when configuring notifications

If a project manually configures the oslo.messaging transport url for
notifications it should use 'get_notification_url', not
'get_transport_url'.  get_transport_url should only be used to obtain
the RPC transport address.

Change-Id: I77772dfa9f30a3db2db6d0387260dfe3452a26ef
Closes-Bug: #1708754
2 files changed