Extend MachineStatus to add ProviderID (#565)

pkg/apis/cluster/v1alpha1/machine_types.go

@@ -78,6 +78,19 @@ type MachineSpec struct {
 	// as-is.
 	// +optional
 	ConfigSource *corev1.NodeConfigSource `json:"configSource,omitempty"`
+
+	// ProviderID is the identification ID of the machine provided by the provider.
+	// This field must match the provider ID as seen on the node object corresponding to this machine.
+	// This field is required by higher level consumers of cluster-api. Example use case is cluster autoscaler
+	// with cluster-api as provider. Clean-up login in the autoscaler compares machines v/s nodes to find out
+	// machines at provider which could not get registered as Kubernetes nodes. With cluster-api as a
+	// generic out-of-tree provider for autoscaler, this field is required by autoscaler to be
+	// able to have a provider view of the list of machines. Another list of nodes is queries from the k8s apiserver
+	// and then comparison is done to find out unregistered machines and are marked for delete.
+	// This field will be set by the actuators and consumed by higher level entities like autoscaler  who will
+	// be interfacing with cluster-api as generic provider.
+	// +optional
+	ProviderID *string `json:"providerID,omitempty"`
 }
 
 /// [MachineSpec]