let's make this an extension with it's own file. I think it's best to provide a high level of visibility when you monkey patch and to be consistent with it.

I'd create the files:
- `lib/berkshelf/vagrant_ext.rb`: a require farm for `lib/berkshelf/vagrant_ext/*`. See https://github.com/RiotGames/berkshelf/blob/master/lib/berkshelf/core_ext.rb for an example
- `lib/berkshelf/vagrant_ext/ui.rb`: contains the monkey patches for the `Vagrant::UI` class