Make authn_passthru work again

Apache authentication methods have been broken since the website was
re-written with Django. While authn_pam handles most cases, it cannot
handle single-sign-on using an existing Kerberos TGT, but this case
is well handled by mod_auth_kerb in Apache. So, it is useful to
make authn_passthru work.

As before, this mode is triggered by setting the authentication
module in modules.conf = 'authn_passthru'. In this mode, the
login form is bypassed, REMOTE_USER *must* be passed in by Apache, and
cobbler does no further checking for authentication.