Don't fail on warnings in docker-cmd

It seems that during the undercloud upgrade heat-config can be empty,
and if it is, the heat-config-docker-cmd hook returns 1 and makes
os-refresh-config fail, consequently failing the undercloud upgrade.

Change-Id: Ia62e40e05f46e7e4450c6b230ddeec9b861e99bd
Related-Bug: #1648249
Closes-Bug: #1692899

heat-config-docker-cmd/os-refresh-config/configure.d/50-heat-config-docker-cmd

@@ -39,13 +39,13 @@ def main(argv=sys.argv):
 
     if not os.path.exists(CONF_FILE):
         log.warning('No config file %s' % CONF_FILE)
-        return 1
+        return 0
 
     try:
         configs = json.load(open(CONF_FILE))
     except ValueError as e:
         log.warning('Could not load config json: %s' % e)
-        return 1
+        return 0
 
     cmd_config_ids = [c['id'] for c in configs
                       if c['group'] == 'docker-cmd']