This adds a check to the `#wait_for_reboot` method on the linux
guest reboot capability to determine if the a reboot is still
in process. This prevents the reboot process from being initiated
and the `#ready?` check on the guest being called before the
system shutdown process has shutdown the communicator process.
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.