Provide explicit image create value in Registry v2 API test

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

Closes-Bug: #1279753
Change-Id: Ia1957e8c024f81eeaa3c0bc094dd62db1be52d07
Signed-off-by: Zhi Yan Liu <zhiyanl@cn.ibm.com>
Merge "Provide explicit image create value in Registry v2 API test"