The validity of "expires_at" parameter should be checked when creating trust

[Liusheng]

The keystone has no check about the validity of "expires_at" when creating a trust. like this:
{
    "trust": {
        "expires_at": "2014-12-30T23:59:59.999999Z",
        "impersonation": false,
        "project_id": "30be1fc9a03c4e94ab05c403a8a377f2",
        "roles": [
            {
                "name": "admin"
            }
        ],
        "trustee_user_id": "58ab3b57493a4a15aada649359493215",
        "trustor_user_id": "e1d870e51c7340cb9d555b15cbfcaec2"
    }
}

The "expires_at" should be later than now.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/188315

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/188315
Committed: https://git.openstack.org/cgit/openstack/keystone/commit/?id=345d5a9f1914308ff894180dbc3b732f3860bf2b
Submitter: Jenkins
Branch: master

commit 345d5a9f1914308ff894180dbc3b732f3860bf2b
Author: liu-sheng <email address hidden>
Date: Thu Jun 4 17:06:09 2015 +0800

    Add validity check of 'expires_at' in trust creation

    It is meaningless that creating a trust with expiration older than now.
    This change add the validity check of "expires_at" when creating a trust.

    Change-Id: Iebdfe86721204a550a74a00c46d661d903a4a141
    Closes-Bug: 1456069