Add config item for secret to avoid service refresh

When CouchDB starts, if it doesn't have a value for 'secret', it
will create one.  Subsequent Puppet runs will see a difference,
overwrite the config, and restart the service.

The value isn't used for anything in the default config, and is
documented at:
http://couchdb.readthedocs.org/en/latest/config/auth.html#couch_httpd_auth/secret