Fixes a race condition in the fixture of test_delete_image

* Added an extra step to the test_delete_image fixture to wait
  for a server's task state to report as none before continuing.
  When a snapshot completes, a server may briefly report as
  snapshotting still, which was causing pseduo-random failures

Change-Id: Ic543e521d627f42f2f61174b6e1bc8f3725dc8b8
Merge "Fixes a race condition in the fixture of test_delete_image"