objects: don't allow to update create_at and update_at

Those fields are managed by sqlalchemy layer, we should not allow to
update them via OVO.

Partially-Implements: blueprint adopt-oslo-versioned-objects-for-db
Change-Id: Ica8f50c45eb482e7a48d9b5b2b997cf9133b3100

neutron/objects/extensions/standardattributes.py

@@ -25,3 +25,5 @@ def add_standard_attributes(cls):
     # its own instance of list
     cls.fields = cls.fields.copy()
     cls.fields.update(STANDARD_ATTRIBUTES)
+    # those fields are updated by sqlalchemy itself
+    cls.fields_no_update += ('created_at', 'updated_at')