crash shell: Remove default user/pass auth

Remove the default availability of user/password login.

Also, change the location of the default SSH host key. It's not saved between
sessions because /etc/ssh isn't created in the image.

Fixes #63.

Signed-off-by: Or Cohen <orc@fewbytes.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>