Rewrite without using any Berkshelf code

Should just shell out to whatever berks you have installed.

@samdunne still interested? Maybe we can pair on this next week.
I think we should say 

> In order to use the Vagrant-Berkshelf plugin, you must have ChefDK installed. To download the latest ChefDK, visit http://getchef.com/downloads/chef-dk. 

As it's written, it makes it sound like vagrant-berkshelf is a "step", which doesn't make sense to me