add open-ended-message to XQUEUE_QUEUES

due to https://github.com/edx/edx-platform/blob/master/common/lib/xmodule/xmodule/open_ended_grading_classes/openendedchild.py#L79

I added  open-ended-message to XQUEUE_QUEUES

playbooks/roles/xqueue/defaults/main.yml

@@ -9,6 +9,7 @@ XQUEUE_QUEUES:
   'test-pull': !!null
   'certificates': !!null
   'open-ended': !!null
+  'open-ended-message': !!null
 XQUEUE_LOGGING_ENV: sandbox
 XQUEUE_SYSLOG_SERVER: 'localhost'
 XQUEUE_S3_BUCKET : 'sandbox-bucket'