(GH-320) Allow disabling of vbguest plugin auto-update

A common Vagrant plugin can cause considerably lengthier test runs
unless disabled in the Vagrantfile.

This patch allows disabling the "vb auto update plugin" via an
environment variable

Originally coded by @petems (Peter Souter)
  refactored by @justinstoller (Justin Stoller)
Merge pull request #525 from justinstoller/GH-320-disable_vbguest_plugin

(GH-320) Allow disabling of vbguest plugin auto-update