Password less sudo does not work for the 'vagrant' user

I enjoy using your vagrant boxes and like the fact that you have published the scripts used to bootstrap them.

However today I realised that password less sudo did not work in the box, which apparently is against Vagrant's conventions.

#### reproduction steps
  * build a box, using master
  * extract the box (`tar -xf [boxname]`) and import it into your vm software
  * start the vm, log in with `vagrant/vagrant`
  * `sudo su`: it ask for a password

#### fix
  * add `vagrant` user to the `vagrant` group which was setup for password less sudo

This change works for me on OS X 10.9/Virtualbox 4.3.0.