Use mod_auth_gssapi instead of mod_auth_kerb.
    
    https://fedorahosted.org/freeipa/ticket/4190
    
    Reviewed-By: Jan Cholasta <jcholast@redhat.com>
    Reviewed-By: Petr Vobornik <pvoborni@redhat.com>
    Reviewed-By: Rob Crittenden <rcritten@redhat.com>
    Reviewed-By: Simo Sorce <ssorce@redhat.com>