replica install: respect --pkinit-cert-file
When --pkinit-cert-file is used, make sure the certificate and key is
actually passed to `KrbInstance`.
https://pagure.io/freeipa/issue/6831
Reviewed-By: Stanislav Laznicka <slaznick@redhat.com>
Reviewed-By: Martin Babinsky <mbabinsk@redhat.com>