This works by registering a `config` with `:provider => true` with the
same name as your provider. Vagrant will then automatically configure
the provider when `config.vm.provider` is used.
This will eventually replace the Environment#vms method. Because of the
introduction of providers, the environment doesn't know what the backing
of the machines will be (and they're _machines_ now, not _vms_).
Instead, users of Environment will now call `#machine` on the
environment to retrieve a machine with the given backing provider as it
needs it.
Mitchell Hashimoto