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>