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
08a6254348