build-vagrant-box improvements

Refs #258, #180, #144.
Fixes #791, #827, #835.

- add proper (`t.p.usage`) command-line parsing
- allows specifying the buildserver to use
- updates the base box before building
- fixes the version selection (our RPM version numbers changed, recently)
It seems a bit strange to declare `__all__` in a private module. Perhaps this should be public or perhaps those variables should be exposed as `admin.TOPLEVEL` etc.