ipnet: add marker for controller-gen

Needed to tell controller-gen we're implementing our own marshalling.
Otherwise it fails for newer versions of controller-gen with

```
/go/src/github.com/openshift/installer/pkg/ipnet/ipnet.go:18:2: encountered struct field "" without JSON tag in type "IPNet"
```

Author: r4f4

pkg/ipnet/ipnet.go

@@ -14,6 +14,7 @@ var nullBytes = []byte(nullString)
 var emptyIPNet = net.IPNet{}
 
 // IPNet wraps net.IPNet to get CIDR serialization.
+// +kubebuilder:validation:Type=string
 type IPNet struct {
 	net.IPNet
 }