Use sudoers.d in Debian

Debian 6.0+ have sudoers.d enabled by default. Use it so that the
provisioners (like Chef) don't need to agree on using "sudo" group, or
know anything about "vagrant" user if it manages the /etc/sudoers file.
Merge pull request #163 from tmatilai/debian-sudoers

Use sudoers.d in Debian