Multi-VM workaround for `berkshelf-vagrant` plugin

Currently `berkshelf-vagrant` doesn't work correctly with multi-VM 
Vagrantfiles. The current workaround is to add a 
`config.berkshelf.berksfile_path` to each vm.define block.

More info here:
https://github.com/RiotGames/berkshelf/issues/436#issuecomment-15394731
https://github.com/RiotGames/berkshelf-vagrant/issues/4#issuecomment-15979246