packaging: setup: do not use private key when constructing ssh public

configuration can have ssh-askpass which is in conflict to us.

ssh-keygen of rhel does not have -m PKCS8 so we must construct the
public key format our-selves.

Change-Id: Ia79ea31d259003a880f0a01b6f888fd3e7bc993f
Signed-off-by: Alon Bar-Lev <alonbl@redhat.com>