clean up xqueue callback url generation; allow override from settings

lms/djangoapps/courseware/module_render.py

@@ -177,18 +177,13 @@ def get_module_for_descriptor(user, request, descriptor, model_data_cache, cours
     # Intended use is as {ajax_url}/{dispatch_command}, so get rid of the trailing slash.
     ajax_url = ajax_url.rstrip('/')
 
-    # Fully qualified callback URL for external queueing system
-    xqueue_callback_url = '{proto}://{host}'.format(
-        host=request.get_host(),
-        proto=request.META.get('HTTP_X_FORWARDED_PROTO', 'https' if request.is_secure() else 'http')
-    )
-
     def make_xqueue_callback(dispatch='score_update'):
         # Fully qualified callback URL for external queueing system
         xqueue_callback_url = '{proto}://{host}'.format(
             host=request.get_host(),
             proto=request.META.get('HTTP_X_FORWARDED_PROTO', 'https' if request.is_secure() else 'http')
         )
+        xqueue_callback_url = settings.XQUEUE_INTERFACE.get('callback_url',xqueue_callback_url)	# allow override
 
         xqueue_callback_url += reverse('xqueue_callback',
                                       kwargs=dict(course_id=course_id,