This branch brings in a whole lot of awesome. The name does not do it
justice. The list of things that comes into play here:
* "virtualbox" is no longer hardcoded anywhere in core. It is the default
provider, yes, but it is 100% possible now to slip in another provider
and have it work.
* `vagrant up --provider` is a thing. This allows you to specify an
alternate provider. Note that the other commands don't support
`--provider` yet so its not THAT useful, but its getting really close.
* True V2 configuration is in place. That means that `Vagrant.configure`
calls now are loading a completely new configuration version, and old
1.0.x Vagrantfiles are V1 configuration. V1 configuration is upgraded
automatically internally, so backwards compatibility is maintained.
Magic, people, magic.
* `config.vm.provider` is the major new configuration option. This is
how provider-specific configuration will be done. For example, Vagrant
has always provided a way to make a pass of `VBoxManage` calls to
customize your VM via `config.vm.customize` in V1. This now exists
as a VirtualBox configuration option. See the example here:
https://gist.github.com/98f5a0df6a05286dfb73
* Unit tests no longer depend on VirtualBox being installed, because for
unit tests we slip in a "no-op" provider, which is a fully valid
Vagrant provider plug-in that does... NOTHING! Brilliant!
* Lots of core middleware executor improvements that make writing and
using middleware stacks a lot more enjoyable. Enjoy a set of "standard
library middlewares" provided by Vagrant in Vagrant::Action::Builtin.
The multi-provider is really shaping up here.
8cc4910fa9