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)
I'm not going to do this, since `_preamble` has side-effects on `sys.path`. It shouldn't effect `sys`, but it is done first deliberately.