Move common wait_for_image_status from compute images_client to waiters

The wait_for_image_status method in the compute json/xml images_clients
was copy/paste and needed a better error message when it times out, so
rather than write a better error message and copy it in both places,
move the common code into waiters and make the json/xml clients call
that instead, like we do for wait_for_server_status.

The improved error message should help in debugging that failure when
it's hit.

Related-Bug: #1260982

Change-Id: I5e3e33310a91da71467fa744972f1a0e4c0bdb50
Merge "Move common wait_for_image_status from compute images_client to waiters"