auto install vagrant env plugin

[zeroedin]

It is possible to auto install vagrant plugins, look into the possibility of adding this code to reduce user install commands:

unless Vagrant.has_plugin?('vagrant-s3auth')
  # Attempt to install ourself. Bail out on failure so we don't get stuck in an
  # infinite loop.
  system('vagrant plugin install vagrant-env') || exit!

  # Relaunch Vagrant so the plugin is detected. Exit with the same status code.
  exit system('vagrant', *ARGV)
end