[ngs_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: I3b450b0a77080c3489636f753943c70167d4b90b

Author: kajinamit

networking_generic_switch/config.py

@@ -20,6 +20,7 @@
 
 coordination_opts = [
     cfg.StrOpt('backend_url',
+               secret=True,
                help='The backend URL to use for distributed coordination.'),
     cfg.IntOpt('acquire_timeout',
                min=0,