buildserver: replace custom code with python-vagrant

buildserver: replace custom code with python-vagrant

I ran into some annoying issues with UTF-8 output in the vagrant logs, and
it was hard to solve.  So I switched to using python-vagrant, which handles
it all for us.  Its been around since 2012, has a number of contributors,
and is still actively maintained, so it seems like a good bet. I also
packaged it for Debian, including a backport in jessie-backports.

On Debian/jessie, do `apt-get install python3-vagrant/jessie-backports`