move OS detection logic out of the Vagrantfile
vagrant: support colab on /vagrant/colab

This covers the use case of colab being alongside other software in the
root of a larger project.
Avoid CentOS downloading stuff from random mirrors

this completely breaks any attempt of using a HTTP proxy.
Vagrantfile: ask about which box to use
Merge pull request #48 from terceiro/pypi-subproject

Improvements to the pypi branch