Disallow password based access

Totally opinionated view on the default CoreOS sshd_config. Disallow password based access by default.

As root, the system wouldn't let me modify `/etc/ssh/sshd_config` (symlinked to `/usr/share/ssh/sshd_config`). I ended up having to remove the symlink and write my own config file. If there is a better way to manage security settings/config on a CoreOS system, I'm all ears.