Don't download dirty RPMs.

The buildserver doesn't build dirty RPMs so don't try to download them.
We check this here, rather than in `admin/build-vagrant-box`, since the
`Vagrantfile`s may be changed.
Fix TOPLEVEL calculation in admin/_preamble.py.
Factor out vagrant into a module.
Add some tests.
Pass build server in to bootstrap.py.
Handle version via options.
Allow specifying build server.
Update documents to use new options.