Don't set an initial null root password for Amazon / VirtualBox images

A null password allows logging into local PAM services such as "login"
(agetty) and KDM.  That's not actually a security problem for EC2
machines, since they do not have "local" logins; for VirtualBox
machines, if you local access, you can do anything anyway.  But it's
better to be on the safe side and disable password-based logins for
root.