Helpers for updating VirtualBox guest additions

This change automates much of the process of keeping VirtualBox guest additions
up-to-date.

* In Vagrantfile, get host's VirtualBox version and add it as a custom fact.
* Write host's VirtualBox version to '/etc/virtualbox-version' on guest.
* Upon starting new interactive shells, check if VirtualBox guest additions are
  missing or out-of-date. If so, prompt user to update.
* Install kernel headers and build-essential, since they are prerequisite for
  building new versions of guest additions.
* Provide a script, 'update-guest-additions', that downloads and installs the
  correct version of VirtualBox guest additions.

Change-Id: I95fca5bd4692eee55de72f82af3035cbd0d3ac6f