Keep current and previous snapshot images

The previous logic around how to keep images was not accomplishing
anything particularly useful.

Instead:
  * Delete images that are not configured or have no corresponding
    base image.
  * Keep the current and previous READY images.
  * Otherwise, delete any images that have been in their current
    state for more than 8 hours.

Also, correct the image-update command which no longer needs to
join a thread.  Also, fix up some poorly exercised parts of the
fake provider.

Change-Id: Iba921f26d971e56692b9104f9d7c531d955d17b4