(WIP) Garbage Collection - Free image space

Reading this block it seems like we only remove the images when we are at a diskspace threshold but in [removeUnusedImages](https://github.com/taskcluster/docker-worker/pull/44/files#diff-19a040dc3a0556202f033ff5cef2f81aR141) we only allow removals after the expiration (which means we may still fail to free up the needed space). The second case here is when we are below the quote where we don't run the cleanup.

I think the following makes more sense:

  - when we hit the threshold delete any images (regardless of time) that are not running (if we feel fancy we can stop deleting once we freed up enough space
  - when we are below the threshold delete only images which are "expired"