default stack_tenant related changes

Have replaced all the 'admin' tenant hardcodings with self.inputs.stack_tenant
Did a comparison of sanity_runs between base and current and didnt find any new failures.
I believe your query is like what if VN has policy1(not None) initially and then another policy gets appended. In such a case, yes this will break unless the caller sets the policy_objs to None before calling the update().

Found an instance in vpc_vn_fixture where we do update policies after VN creation(however here policy_objs will be None so we will not have issues). 
Hate to add self.scale check everywhere however will add the same here and update the diff.