kubeadm: disallow the mixture of --config and --certificate-key

Similar to --token, do not allow the mixture of --config and
--certificate-key.

If the user has fed a config, it is expected that the certificate
key should also be provided in the config and not from
the command line.

cmd/kubeadm/app/apis/kubeadm/validation/validation.go

@@ -433,7 +433,6 @@ func isAllowedFlag(flagName string) bool {
 		kubeadmcmdoptions.KubeconfigDir,
 		kubeadmcmdoptions.UploadCerts,
 		kubeadmcmdoptions.ExperimentalUploadCerts,
-		kubeadmcmdoptions.CertificateKey,
 		"print-join-command", "rootfs", "v")
 	if knownFlags.Has(flagName) {
 		return true