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.
Vagrant Core Plugins
These are plugins that ship with Vagrant. Vagrant core uses its own plugin system to power a lot of the core pieces that ship with Vagrant. Each plugin will have its own README which explains its specific role.