Utilize implemented functions

pkg/upgrader/upgrade/upgrade.go

@@ -7,18 +7,28 @@ import (
 )
 
 const (
-	labelUpgradeLock      = "kubeone.io/upgrading-in-process"
+	labelUpgradeLock      = "kubeone.io/upgrade-in-progress"
 	labelControlPlaneNode = "node-role.kubernetes.io/master"
 )
 
 // Upgrade performs all the steps required to upgrade Kubernetes on
 // cluster provisioned using KubeOne
 func Upgrade(ctx *util.Context) error {
-	if err := util.BuildKubernetesClientset(ctx); err != nil {
-		return errors.Wrap(err, "unable to build kubernetes clientset")
+	upgradeSteps := []struct {
+		fn     func(ctx *util.Context) error
+		errMsg string
+	}{
+		{fn: util.BuildKubernetesClientset, errMsg: "unable to build kubernetes clientset"},
+		{fn: determineHostname, errMsg: "unable to determine hostname"},
+		{fn: runPreflightChecks, errMsg: "preflight checks failed"},
+		{fn: upgradeLeaderControlPlane, errMsg: "unable to upgrade the leader control plane node"},
+		{fn: upgradeFollowerControlPlane, errMsg: "unable to upgrade follower control plane node"},
 	}
-	if err := runPreflightChecks(ctx); err != nil {
-		return errors.Wrap(err, "preflight checks failed")
+
+	for _, step := range upgradeSteps {
+		if err := step.fn(ctx); err != nil {
+			return errors.Wrap(err, step.errMsg)
+		}
 	}
 
 	return nil