Set celery priority in supervisord higher

From supervisord [docs](http://supervisord.org/configuration.html):

> Higher priorities indicate programs that start last and shut down first.

extra/supervisord/celeryd.conf

@@ -29,6 +29,6 @@ stopwaitsecs = 600
 ; taking care of its children as well.
 killasgroup=true
 
-; if rabbitmq is supervised, set its priority higher
-; so it starts first
-priority=998
+; Set Celery priority higher than default (999)
+; so, if rabbitmq is supervised, it will start first.
+priority=1000