WIP fix for Vagrant 1.5

This is a WIP branch to show how vagrant-berkshelf can be fixed for Vagrant 1.5 (#156)

One breaking *internal* changes in Vagrant 1.5 is that `env[:global_config]` is removed. It looks like `env.vagrantfile` is added for that purpose.

I could run Vagrant 1.5 with this branch successfully, but I do not know all internals of vagrant-berkshelf, so I'm not sure if there are still any other broken things.