WIP: add ci-fairy command to delete dangling job tags

ci-fairy delete-images --unused-jobtags will delete any foo-built-by-job-1234 tag whose revision does not match that of the tag foo.

Test run from git tree with:

$  podman login -u $USER registry.freedesktop.org
# password is the API token you just generated in your user profile with scope api
$ python3 ./tools/ci_fairy.py -vv delete-image --project $USER/mesa --dry-run --unused-jobtag

You really don't want to forget the --dry-run ;)

cc @daenzer