fix NameError and AttribteError
diff --git a/planetstack/observer/steps/sync_slivers.py b/planetstack/observer/steps/sync_slivers.py
index 74a1b50..900840d 100644
--- a/planetstack/observer/steps/sync_slivers.py
+++ b/planetstack/observer/steps/sync_slivers.py
@@ -1,5 +1,6 @@
 import os
 import base64
+from django.db.models import F, Q
 from planetstack.config import Config
 from observer.openstacksyncstep import OpenStackSyncStep
 from core.models.sliver import Sliver