Steps in which deletion is not supported have to return an empty list
when queried for deleted objects.
diff --git a/planetstack/openstack_observer/steps/sync_sliver_ips.py b/planetstack/openstack_observer/steps/sync_sliver_ips.py
index e2212d1..bd1d572 100644
--- a/planetstack/openstack_observer/steps/sync_sliver_ips.py
+++ b/planetstack/openstack_observer/steps/sync_sliver_ips.py
@@ -9,7 +9,10 @@
     provides=[Sliver]
     requested_interval=0
 
-    def fetch_pending(self):
+    def fetch_pending(self, deleted):
+        # Not supported yet
+        if (deleted):
+            return []
         slivers = Sliver.objects.filter(ip=None)
         return slivers