use env[:machine] for vagrant enable/disable

this allows you to specify on a per box basis if berkshelf is enabled or disabled

This should address issues #72 and allow you to disable berkshelf per vm definition.

With this change you can now do 

```ruby
Vagrant.configure("2") do |config|
  config.vm.define "foo" do |cfg|
    cfg.berkshelf.enabled = false
    ...
  end
end
```

/cc @claco