Make ClusterNetwork ClusterNetworkingConfig field optional within (ku…

…bernetes-sigs#919)

Cluster resources. This is useful for Managed Control Planes and
Bare Metal where networking configuration options may be limited.

(cherry picked from commit 9e1eb56)

pkg/apis/cluster/v1alpha1/cluster_types.go

@@ -47,6 +47,7 @@ type Cluster struct {
 // ClusterSpec defines the desired state of Cluster
 type ClusterSpec struct {
 	// Cluster network configuration
+	// +optional
 	ClusterNetwork ClusterNetworkingConfig `json:"clusterNetwork"`
 
 	// Provider-specific serialized configuration to use during