the update time is not updated when update zone of aggregate

[Summary]
the update time is not updated when update zone of aggregate

[Topo]
devstack all-in-one node

[Description and expect result]
the update time is updated when update zone of aggregate

[Reproduceable or not]
reproduceable

[Recreate Steps]
1) the update time is not updated when update zone of aggregate:
root@45-59:~# openstack aggregate set --zone "AB" agg1
+-------------------+---------------------------------------------+
| Field | Value |
+-------------------+---------------------------------------------+
| availability_zone | AB |
| created_at | 2016-01-26T11:34:00.000000 |
| deleted | False |
| deleted_at | None |
| hosts | [] |
| id | 5 |
| metadata | {u'abc': u'1', u'availability_zone': u'AB'} |
| name | agg1 |
| updated_at | None >>>ISSUE |
+-------------------+---------------------------------------------+
root@45-59:~# openstack aggregate set --zone "ab" agg1
+-------------------+---------------------------------------------+
| Field | Value |
+-------------------+---------------------------------------------+
| availability_zone | ab |
| created_at | 2016-01-26T11:34:00.000000 |
| deleted | False |
| deleted_at | None |
| hosts | [] |
| id | 5 |
| metadata | {u'abc': u'1', u'availability_zone': u'ab'} |
| name | agg1 |
| updated_at | None >>>ISSUE |
+-------------------+---------------------------------------------+

[Configration]
reproduceable bug, no need

[logs]
reproduceable bug, no need

[Root cause anlyze or debug inf]
reproduceable bug

[Attachment]
None