[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

Change-Id: I30e796a81fe0bb58023b98ffd10f7db269518ac5

vitrage/coordination/__init__.py

@@ -16,6 +16,7 @@ from oslo_config import cfg
 
 OPTS = [
     cfg.StrOpt('backend_url',
+               secret=True,
                help='The backend URL to use for the coordination service. If '
                     'left empty, membership api will not work')
 ]