fix queryset.remove bug in sync step

Change-Id: I5f9a3c487c70db349b89d8e294945a41082843b2
diff --git a/xos/synchronizer/steps/sync_vroutertenant.py b/xos/synchronizer/steps/sync_vroutertenant.py
index 190dd2b..3fa95a4 100644
--- a/xos/synchronizer/steps/sync_vroutertenant.py
+++ b/xos/synchronizer/steps/sync_vroutertenant.py
@@ -43,6 +43,8 @@
         else:
             objs = VRouterTenant.get_deleted_tenant_objects()
 
+        objs = list(objs)
+
         # Check that each is a valid vCPE tenant or instance
         for vroutertenant in objs:
             # Do we have a vCPE subscriber_tenant?