certs: do not re-create NSS database when requesting service cert
    
    `CertDB.request_service_cert` could re-create NSSDB files if the supplied CA
    certificate was not found in database. This could cause subtle bugs since the
    files were recreated with wrong permissions. This behavior was removed so that
    there are no destructive operations performed by the method.
    
    https://fedorahosted.org/freeipa/ticket/6429
    
    Reviewed-By: Stanislav Laznicka <slaznick@redhat.com>