HTCONDOR-829: More work on getting image versions from Git tags
HTCONDOR-829: More work on getting image versions from Git tags

Other features added:
- a --one-tag flag for only creating the most specific tag for each
  image (e.g. '9.11.0-el7' but not '9.x-el7')
- 'lts' and 'lts-DISTRO' image tags for the latest LTS release

Also this drops support for creating images for the 8.9 series since
that series is unsupported anyway, and I can save code by not having
to deal with two versioning schemes.

htcondor/htcondor,htcondor/htcondor,htcondor/htcondor,htcondor/htcondor,htcondor/htcondor,htcondor/htcondor,htcondor/htcondor,htcondor/htcondor