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
Merge pull request #1461 from fmyzjs/patch-4

add open-ended-message to XQUEUE_QUEUES