This works by now calling the `:ssh` action on the provider. This action
is allowed to do whatever it pleases, but should at some point probably
call the `SSHExec` built-in middleware.
The `SSHExec` built-in middleware was added. This uses the information
returned by `Machine#ssh_info` and uses the `Vagrant::Util::SSH` helper
to exec into the remote machine. The provider should do any work upfront
in verifying that the machine is ready to be SSHed into.
This starts the transition of replacing VM with Machine. Machine still
isn't ready to fully replace VM but by moving it now, I'm able to find
the spots that need to be fixed. At this point `vagrant status` works
with the new provider interface.
