refactor(contrib): refactor contrib READMEs

The main README should describe our supported providers, and Vagrant-specific
stuff should live in /contrib.

This commit does several things:
* Moves Vagrantfile and Makefile into contrib/vagrant-1-node
* Moves Vagrant-specific commands and instructions to contrib/vagrant-1-node/README.md
* Renames contrib/vagrant to contrib/vagrant-n-node
* Rewords various READMEs to be more clear