On demand resources, started via on-demand tags

On-demand tickets need to be prioritized over the normal tickets
(priority queue), otherwise we risk that normal tickets take "on demand"
resources.

If we want to prefer one "on demand" pool over the other "on demand"
(e.g. spot AWS over normal instances), we need to sort them through a
priority queue.  The fallback (if SPOT instances don't start) isn't
resolved yet, though, we just prioritize per tag priority as usually.

resallocserver/logic.py

@@ -69,10 +69,11 @@ def ready(self):
         """
         Get ready resources, those which were never assigned or are released.
         The sandbox-assigned resources are sorted above others - so they can be
-        re-used first.
+        re-used first.  The query is already ordered by ID ASC.
         """
         return (self.up().filter(models.Resource.ticket_id.is_(None))
-                         .filter(models.Resource.check_failed_count==0))
+                         .filter(models.Resource.check_failed_count==0)
+                         .order_by(models.Resource.id.asc()))
 
     def taken(self):
         """