After creating a snapshot of a running instance, a print out similar
to server create is expected, but it prints out something like "_info"
which is nothing related to created image. _prep_image_detail method
is added to /compute/v2/server.py to enable the priting, while running
the test properly.
Change-Id: I4b06be959768bcdaafd9aa8df497490958bee649
Closes-Bug:1551586