Add REST PrepareForUpdate() hook

As per discussion with @snmarterclayton.  I implemented this for most
types in the "obvious" way.  I am not sure how to implement
this for a couple types, though.

pkg/api/rest/types.go

@@ -66,6 +66,14 @@ func (svcStrategy) PrepareForCreate(obj runtime.Object) {
 	service.Status = api.ServiceStatus{}
 }
 
+// PrepareForUpdate clears fields that are not allowed to be set by end users on update.
+func (svcStrategy) PrepareForUpdate(obj, old runtime.Object) {
+	// TODO: once service has a status sub-resource we can enable this.
+	//newService := obj.(*api.Service)
+	//oldService := old.(*api.Service)
+	//newService.Status = oldService.Status
+}
+
 // Validate validates a new service.
 func (svcStrategy) Validate(obj runtime.Object) fielderrors.ValidationErrorList {
 	service := obj.(*api.Service)

pkg/api/rest/update.go

@@ -33,6 +33,10 @@ type RESTUpdateStrategy interface {
 	NamespaceScoped() bool
 	// AllowCreateOnUpdate returns true if the object can be created by a PUT.
 	AllowCreateOnUpdate() bool
+	// PrepareForUpdate is invoked on update before validation to normalize
+	// the object.  For example: remove fields that are not to be persisted,
+	// sort order-insensitive list fields, etc.
+	PrepareForUpdate(obj, old runtime.Object)
 	// ValidateUpdate is invoked after default fields in the object have been filled in before
 	// the object is persisted.
 	ValidateUpdate(obj, old runtime.Object) fielderrors.ValidationErrorList
@@ -53,6 +57,7 @@ func BeforeUpdate(strategy RESTUpdateStrategy, ctx api.Context, obj, old runtime
 	} else {
 		objectMeta.Namespace = api.NamespaceNone
 	}
+	strategy.PrepareForUpdate(obj, old)
 	if errs := strategy.ValidateUpdate(obj, old); len(errs) > 0 {
 		return errors.NewInvalid(kind, objectMeta.Name, errs)
 	}