ajacoutot spotted a problem with the new sshd logic (to disable root logins

ajacoutot spotted a problem with the new sshd logic (to disable root logins
by default completely in most cases, except where a public ssh key was provided
to autoinstall) - in the case where a (non-root) account was created, sshd
was being disabled; this diff fixes it. Looks good ajacoutot, OK djm@,
extensive testing+OK rpe@,