CORD-2329: Constraint node assignments at a serviceinstance level

Change-Id: I84995def913733a137afb6d746e076cf622d6056
diff --git a/xos/synchronizer/model_policies/model_policy_venbserviceinstance.py b/xos/synchronizer/model_policies/model_policy_venbserviceinstance.py
index 9af1b80..ae8635e 100644
--- a/xos/synchronizer/model_policies/model_policy_venbserviceinstance.py
+++ b/xos/synchronizer/model_policies/model_policy_venbserviceinstance.py
@@ -19,6 +19,7 @@
 
 class VENBServiceInstancePolicy(TenantWithContainerPolicy):
     model_name = "VENBServiceInstance"
+    constrain_by_service_instance = True
 
     def handle_create(self, service_instance):
         return self.handle_update(service_instance)