ovo: Make return values from objects db api consistent

Previously in db api, we returned dictionaries from create() and update()
methods while get() methods returned db models. This patch makes the
return values consistent and thus we always get db model from db api
operations.

Related-Bug: 1541928
Change-Id: I7e9ec4efa20e3d09e1d5a0ca04799095b6ea42b7

neutron/objects/base.py

@@ -213,7 +213,8 @@ class NeutronDbObject(NeutronObject):
         :param db_obj: dict of object fetched from database
         :return: modified dict of DB values
         """
-        result = dict(db_obj)
+        result = {field: value for field, value in dict(db_obj).items()
+                  if value is not None}
         for field, field_db in cls.fields_need_translation.items():
             if field_db in result:
                 result[field] = result.pop(field_db)

neutron/objects/db/api.py

@@ -37,7 +37,7 @@ def create_object(context, model, values):
             values['id'] = uuidutils.generate_uuid()
         db_obj = model(**values)
         context.session.add(db_obj)
-    return db_obj.__dict__
+    return db_obj
 
 
 def _safe_get_object(context, model, **kwargs):
@@ -55,7 +55,7 @@ def update_object(context, model, values, **kwargs):
         db_obj = _safe_get_object(context, model, **kwargs)
         db_obj.update(values)
         db_obj.save(session=context.session)
-    return db_obj.__dict__
+    return db_obj
 
 
 def delete_object(context, model, **kwargs):