Remove internal Berkshelf workflow and other things...

This pull request contains several changes that attempt to ease the maintainability of this project:

* Replace internal Berkshelf workflow with the [vagrant-berkshelf](https://github.com/riotgames/vagrant-berkshelf) plugin
* Update Vagrant boxes with their provisionerless replacements and take care of provisioner installation (Chef) via [vagrant-omnibus](https://github.com/schisamo/vagrant-omnibus)
* Remove `sudo` cookbook dependency because Vagrant boxes already require the `vagrant` user to have `sudo` capabilities
* Bump Riak cookbook version to `v2.1.0`