mitchellh/vagrant — pull request #3600 by mitchellh (2014-05-01 16:53:57+00:00)
Title: Default Provider By Priority

This introduces smarter `Environment#default_provider` logic which is done by priority and usability of providers. `VAGRANT_DEFAULT_PROVIDER` still trumps the rest, but if that isn't set, Vagrant does its best to choose the
"best" provider.