Provide explicit image create value for test_image_paginate case

Assign an explicit created_at datetime value to image db fixtures,
it be used to fixes race condition in DB driver test_image_paginate
case.

Closes-Bug: #1282400
Change-Id: I359633af4bb25cf1866b42d6dad3df658995b7bd
Signed-off-by: Zhi Yan Liu <zhiyanl@cn.ibm.com>
(cherry picked from commit 1c5797d53a53a990cef3d695f0752742fd213840)