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.
Reviewed-By: Stanislav Laznicka <firstname.lastname@example.org>