Fix some volume testing glitches.

* Give grepable names to volumes. Then have the cleanup script remove stale
  volumes.
* Make the volume context wait for the volume to be available, to avoid
  destroy-too-soon races.
* Swap the order of with contexts, so we don't create volumes needlessly for
  image combos that don't exist.

Change-Id: Ib0a29c5dd5645f66fdc3d94ba526bff7e4cf7b78