return `revoked_at` for the revocation event

[Dave Chen]

Currently, the `revoked_at` is not returned when the API of getting revocation events are executed, the `revoked_at` is always available in the backends, get back this to the end user will help to give us the clue on what happened and when the token was revoked, especially, if the `issued_before` is set with a different time compared with `revoked_at`.

Steps to reproduce,
1. create a token.
2. delete the token.
3. list revoke events.

The sample output as follow,
{"events": [{"issued_before": "2016-06-27T06:12:52.000000Z", "audit_id": "MwPMflshThm7zj7lGcWOnQ"}, {"issued_before": "2016-06-27T07:35:31.000000Z", "audit_id": "nJpSKrzUQLq-jjygqvB-sA"}], "links": {"self": "http://10.239.159.68/identity/v3/OS-REVOKE/events", "previous": null, "next": null}}

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/336439

[OpenStack Infra (hudson-openstack)]

Related fix proposed to branch: master
Review: https://review.openstack.org/336963

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/336439
Committed: https://git.openstack.org/cgit/openstack/keystone/commit/?id=25e5227d23d7b8e1b2969a6b7dd6da4fe50972a4
Submitter: Jenkins
Branch: master

commit 25e5227d23d7b8e1b2969a6b7dd6da4fe50972a4
Author: Dave Chen <email address hidden>
Date: Fri Jul 1 16:40:18 2016 +0800

    Return `revoked_at` for list revoke events

    The field will help to figure out when the event is recorded, and thus
    give a clue on when the revocation was done.

    Change-Id: If5ba3bb9b68fff5f35be2b57af15584d69c6df8d
    Closes-Bug: #1598040

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/336963
Committed: https://git.openstack.org/cgit/openstack/keystone/commit/?id=b425379360438f6fea50d1e69c121ea65827800e
Submitter: Jenkins
Branch: master

commit b425379360438f6fea50d1e69c121ea65827800e
Author: Dave Chen <email address hidden>
Date: Mon Jul 4 08:53:27 2016 +0800

    Refactoring: remove the duplicate method

    The method `assertTimestampEqual` which compare two ISO 8601 format
    time can be used across the testcases, so move the method to core.py
    and remove the duplicate method.

    Change-Id: I7bd34380f87da0ee8a52af228f1cf966d90b33c8
    Related-Bug: #1598040

[Davanum Srinivas (DIMS) (dims-v)]

This issue was fixed in the openstack/keystone 10.0.0.0b2 development milestone.