Code review comment for lp://staging/~pwlars/ubuntu-ci-services-itself/full-er-er-imagebuild-logging

Ok, I think this should handle all your comments. One more thing I'd like to do is add some unit tests for run_worker as there are quite a few more paths through it now. It will have to be heavily mocked of course, but I think it would be worthwhile. On the other hand, I'd like to get this merged sooner than later, since it helps quite a bit with debugging when things go wrong in the image build step. So I added https://bugs.launchpad.net/ubuntu-ci-services-itself/+bug/1288543 to remind me.