Export machine annotation key (#663)

The node controller is not the only consumer of the machine annotation.
Other consumers like cluster autoscaler or controller annotating nodes
with machine reuses the same literal. All consumers should be allow to
import the variable instead of defining the same literal locally.

pkg/controller/node/node.go

@@ -31,10 +31,11 @@ import (
 )
 
 const (
-	machineAnnotationKey = "cluster.k8s.io/machine"
+	// MachineAnnotationKey annotation used to link node and machine resource
+	MachineAnnotationKey = "cluster.k8s.io/machine"
 )
 
-// We are using "machine" annotation to link node and machine resource. The "machine"
+// We are using "cluster.k8s.io/machine" annotation to link node and machine resource. The "cluster.k8s.io/machine"
 // annotation is an implementation detail of how the two objects can get linked together, but it is
 // not required behavior. However, in the event that a Machine.Spec update requires replacing the
 // Node, this can allow for faster turn-around time by allowing a new Node to be created with a new
@@ -52,7 +53,7 @@ func (c *ReconcileNode) link(node *corev1.Node) error {
 		}
 	}
 
-	val, ok := node.ObjectMeta.Annotations[machineAnnotationKey]
+	val, ok := node.ObjectMeta.Annotations[MachineAnnotationKey]
 	if !ok {
 		return nil
 	}
@@ -86,7 +87,7 @@ func (c *ReconcileNode) link(node *corev1.Node) error {
 }
 
 func (c *ReconcileNode) unlink(node *corev1.Node) error {
-	val, ok := node.ObjectMeta.Annotations[machineAnnotationKey]
+	val, ok := node.ObjectMeta.Annotations[MachineAnnotationKey]
 	if !ok {
 		return nil
 	}