Provide explicit image create value in Registry v2 Client test

Assign an explicit created_at datetime value to image db fixtures,
it be used to fixes race condition in Registry v2 Client test cases.

Closes-Bug: #1272136
Change-Id: Ib9f1c7072c6ac828479c6a2e5bddd0f3ad057653
Signed-off-by: Zhi Yan Liu <zhiyanl@cn.ibm.com>
Merge "Provide explicit image create value in Registry v2 Client test"