Add an option to auto-configure Vagrant's chef_solo provisioner

This changeset adds a `provision` option to `Gusteau::Vagrant` which enables auto-configuration of Vagrant's `chef_solo` provider based on your node YAML files.

This adds in a concept of vagrant-only nodes: the ones which have `vagrant` and not `server` in their YAML. 

You can think of it as a Vagrant configuration manager based on Gusteau node YAML files. The idea is to:
1. Be able to use Vagrant's built in capabilities for local nodes
2. Use Gusteau for remote nodes