Make request_service_keytab into a public method
    
    a cosmetic change: we had private method comprising of calls to public
    ones, which did not make much sense in our case
    
    https://fedorahosted.org/freeipa/ticket/6638
    
    Reviewed-By: Martin Basti <mbasti@redhat.com>
    Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>