kubeadm: use current-context when validating kubelet kubeconfig

During initialization `kubeadm init` creates kubelet.conf with
specified name and during finalize phase validates that
this kubeconfig is not corrupted by checking for presence of specific
authinfo

However:
* kubelet doesn't require a specific name for this context
* in external CA mode this kubeconfig can be created outside of
  `kubeadm init`

This change updates kubeadm finalize stage to avoid overly strict
context check.

cmd/kubeadm/app/cmd/phases/init/kubeletfinalize.go

@@ -114,7 +114,17 @@ func runKubeletFinalizeCertRotation(c workflow.RunData) error {
 	}
 
 	// Perform basic validation. The errors here can only happen if the kubelet.conf was corrupted.
-	userName := fmt.Sprintf("%s%s", kubeadmconstants.NodesUserPrefix, cfg.NodeRegistration.Name)
+	if len(kubeconfig.CurrentContext) == 0 {
+		return errors.Errorf("the file %q does not have current context set", kubeconfigPath)
+	}
+	currentContext, ok := kubeconfig.Contexts[kubeconfig.CurrentContext]
+	if !ok {
+		return errors.Errorf("the file %q is not a valid kubeconfig: %q set as current-context, but not found in context list", kubeconfigPath, kubeconfig.CurrentContext)
+	}
+	userName := currentContext.AuthInfo
+	if len(userName) == 0 {
+		return errors.Errorf("the file %q is not a valid kubeconfig: empty username for current context", kubeconfigPath)
+	}
 	info, ok := kubeconfig.AuthInfos[userName]
 	if !ok {
 		return errors.Errorf("the file %q does not contain authentication for user %q", kubeconfigPath, cfg.NodeRegistration.Name)