bug#26695: openssh password-authentication? should be #f by default

[Christopher Allan Webber]

Our default permits password authentication for the openssh service (and
the others it seems) by default in Guix.  This is somewhat dangerous
because this is a much easier to break in this way, and some users might
not assume the default is reasonably safe.  If users really want
password-authentication, they should turn it on explicitly.