Add vagrant user to sudo group instead of admin.

The base distro has replaced the admin group with sudo in the standard
/etc/groups file.

The admin group need not be created if the vagrant user is added to
sudo group instead and /etc/sudoers is edited so that members of sudo
don't require a password.
Merge pull request #467 from lotia/ubuntu-12.04-admin-changes

Add vagrant user to sudo group instead of admin.