The issue here is that when a middleware failed and a recovery sequence
started, it would halt at the "call" step because the "Call" didn't
properly recover the child sequence.
An additional issue was that a Warden had no "recover" method, meaning
embedded Wardens wouldn't recover their stacks properly.
The built-in middleware sequences will now be hardcoded onto
Vagrant::Action. Other plugins can hook into these sequences to provide
verification and so on. So the VirtualBox plugin will hook into that
action sequence and add verification.
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.
