[DEFAULT] periodic_coordinator_backend_url should be secret

This 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.

[DEFAULT]
periodic_coordinator_backend_url=redis://:password@127.0.0.1:6379

Change-Id: I09040b54f6e2ee4f0e71c601400b601b5de19780

sahara/service/periodic.py

@@ -65,6 +65,7 @@ periodic_opts = [
                     'will be deleted automatically. (0 value means that '
                     'automatic clean up is disabled).'),
     cfg.StrOpt('periodic_coordinator_backend_url',
+               secret=True,
                help='The backend URL to use for distributed periodic tasks '
                     'coordination.'),
     cfg.IntOpt('periodic_workers_number',