[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: Ib918532d82c4633add7fad9c8441868cfd84001b

aodh/coordination.py

@@ -28,6 +28,7 @@ LOG = log.getLogger(__name__)
 
 OPTS = [
     cfg.StrOpt('backend_url',
+               secret=True,
                help='The backend URL to use for distributed coordination. If '
                     'left empty, alarm evaluation won\'t do workload '
                     'partitioning and will only function correctly if a '