Use redefined LOW_PRIORITY queue

aws.py redefines the default queues in order to expose cross-environment
work. The new recalculate grades routing key shgould use that value
rather than the default defined in common.py.

lms/envs/aws.py

@@ -273,7 +273,7 @@ BULK_EMAIL_ROUTING_KEY = HIGH_PRIORITY_QUEUE
 BULK_EMAIL_ROUTING_KEY_SMALL_JOBS = LOW_PRIORITY_QUEUE
 
 # Queue to use for updating persistent grades
-RECALCULATE_GRADES_ROUTING_KEY = ENV_TOKENS.get('RECALCULATE_GRADES_ROUTING_KEY', RECALCULATE_GRADES_ROUTING_KEY)
+RECALCULATE_GRADES_ROUTING_KEY = ENV_TOKENS.get('RECALCULATE_GRADES_ROUTING_KEY', LOW_PRIORITY_QUEUE)
 
 # following setting is for backward compatibility
 if ENV_TOKENS.get('COMPREHENSIVE_THEME_DIR', None):