core: VirtualBox is no longer the fallback, not hardcoded anywhere

This commit finally removes all traces of VirtualBox defaults from
Vagrant core. Vagrant now completely relies on its automated provider
detection to pick the best and most relevant provider that is available
on the system and for a specific development environment.

A "vagrant up" on one development environment might prefer VirtualBox,
another might prefer Docker. As a result of this commit (plus the few
prior), the developer doesn't need to know anymore and doesn't need to
specify a `--provider` flag. Vagrant just figures out the correct
provider.

Yay. :)