Sudoer should use secure_path by default

The current sudoer configuration will not work with chef-omnibus install because by default a debian user doesn't have link to sbin.

Debian recommends that you use secure_path (and not exempt_group)
- http://wiki.debian.org/sudo

This change makes it possible for a clean bento build of debian 6.0.7 to be able to install chef-omnibus without having to switch to root or change the PATH variable.