[coordination] backend_url should be secret

The backend_url option can sometimes contain secrets.

For example when redis coordination backend is used and authentication
is enabled in redis, the plain redis password is put as an URL element.

[coordination]
backend_url=redis://:password@127.0.0.1:6379

Closes-Bug: #2012246
Change-Id: Ia815720cedda2f5c70205ffda5c765364cee8f8c
(cherry picked from commit 8ec4687566)
(cherry picked from commit 5da7be2926)
(cherry picked from commit 98ea6ac768)

manila/coordination.py

@@ -30,6 +30,7 @@ LOG = log.getLogger(__name__)
 
 coordination_opts = [
     cfg.StrOpt('backend_url',
+               secret=True,
                default='file://$state_path',
                help='The back end URL to use for distributed coordination.')
 ]