Passwordless sudo as any user

This allows things as `sudo -u postgres` to be done passwordless.

I use Ansible for provisioning which uses `sudo -u` in many patterns. Without this change I have to explicitely supply a password for the `vagrant` user, which is kind of awkward since it has passwordless sudo rights.

The [Vagrant documentation](http://docs.vagrantup.com/v2/boxes/base.html) suggests something similar (look for 'password-less sudo').