Remove opinionated chef install and defer to vagrant-omnibus plugin

@schisamo's [`vagrant-omnibus`](https://github.com/schisamo/vagrant-omnibus) plugin is awesome. Could we not simply add a dependency and do away with the setup in this project?

https://github.com/smdahlen/vagrant-digitalocean/blob/master/lib/vagrant-digitalocean/actions/setup_provisioner.rb

This would also be a plugin-centric way forward for the salt stack install too (Re: #43)