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, making mod_authn
function again is useful.

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. There is no
special case handling required to test that the user is validated
elsewhere, so eliminate that clause from "test_user_authenticated."