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.
This moves out the concept of a "default VM" from the Environment class
and makes it the responsibility of the V1 configuration that at least
one VM is defined on it. This lets the configuration ultimately decide
what a "default" implementation is.
Mitchell Hashimoto