Move defaulting to webhook rather than openapi

Attemping to use openapi defaulting for replicas resulted in the following error:

STEP: bootstrapping test environment
Failure [9.838 seconds]
[BeforeSuite] BeforeSuite
/home/prow/go/src/sigs.k8s.io/cluster-api/bootstrap/kubeadm/api/v1alpha2/suite_test.go:56
  Unexpected error:
      <*errors.StatusError | 0xc0003d4280>: {
          ErrStatus: {
              TypeMeta: {Kind: "", APIVersion: ""},
              ListMeta: {
                  SelfLink: "",
                  ResourceVersion: "",
                  Continue: "",
                  RemainingItemCount: nil,
              },
              Status: "Failure",
              Message: "CustomResourceDefinition.apiextensions.k8s.io \"kubeadmcontrolplanes.cluster.x-k8s.io\" is invalid: spec.validation.openAPIV3Schema.properties[spec].properties[replicas].default: Forbidden: must not be set",
              Reason: "Invalid",
              Details: {
                  Name: "kubeadmcontrolplanes.cluster.x-k8s.io",
                  Group: "apiextensions.k8s.io",
                  Kind: "CustomResourceDefinition",
                  UID: "",
                  Causes: [
                      {
                          Type: "FieldValueForbidden",
                          Message: "Forbidden: must not be set",
                          Field: "spec.validation.openAPIV3Schema.properties[spec].properties[replicas].default",
                      },
                  ],
                  RetryAfterSeconds: 0,
              },
              Code: 422,
          },
      }
      CustomResourceDefinition.apiextensions.k8s.io "kubeadmcontrolplanes.cluster.x-k8s.io" is invalid: spec.validation.openAPIV3Schema.properties[spec].properties[replicas].default: Forbidden: must not be set
  occurred
  /home/prow/go/src/sigs.k8s.io/cluster-api/bootstrap/kubeadm/api/v1alpha2/suite_test.go:66
------------------------------
Ran 1 of 0 Specs in 9.949 seconds
FAIL! -- 0 Passed | 1 Failed | 0 Pending | 0 Skipped
--- FAIL: TestAPIs (9.95s)
FAIL
FAIL	sigs.k8s.io/cluster-api/bootstrap/kubeadm/api/v1alpha2	10.011s

Author: detiber

api/v1alpha3/kubeadm_control_plane_types.go

@@ -28,7 +28,6 @@ type KubeadmControlPlaneSpec struct {
 	// odd numbers are permitted, as per [etcd best practice](https://etcd.io/docs/v3.3.12/faq/#why-an-odd-number-of-cluster-members).
 	// This is a pointer to distinguish between explicit zero and not specified.
 	// +optional
-	// +kubebuilder:default:=1
 	Replicas *int32 `json:"replicas,omitempty"`
 
 	// Version defines the desired Kubernetes version.

api/v1alpha3/kubeadm_control_plane_webhook_default.go

@@ -0,0 +1,33 @@
+/*
+Copyright 2019 The Kubernetes Authors.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package v1alpha3
+
+import (
+	"sigs.k8s.io/controller-runtime/pkg/webhook"
+)
+
+// +kubebuilder:webhook:verbs=create;update,path=/mutate-cluster-x-k8s-io-v1alpha3-kubeadmcontrolplane,mutating=true,failurePolicy=fail,groups=cluster.x-k8s.io,resources=kubeadmcontrolplane,versions=v1alpha3,name=default.kubeadmcontrolplane.cluster.x-k8s.io
+
+var _ webhook.Defaulter = &KubeadmControlPlane{}
+
+// Default implements webhook.Defaulter so a webhook will be registered for the type
+func (r *KubeadmControlPlane) Default() {
+	if r.Spec.Replicas == nil {
+		replicas := int32(1)
+		r.Spec.Replicas = &replicas
+	}
+}

api/v1alpha3/kubeadm_control_plane_webhook.go renamed to api/v1alpha3/kubeadm_control_plane_webhook_validation.go

@@ -861,7 +861,6 @@ spec:
                   type: array
               type: object
             replicas:
-              default: 1
               description: Number of desired machines. Defaults to 1. When stacked
                 etcd is used only odd numbers are permitted, as per [etcd best practice](https://etcd.io/docs/v3.3.12/faq/#why-an-odd-number-of-cluster-members).
                 This is a pointer to distinguish between explicit zero and not specified.

config/crd/bases/cluster.x-k8s.io_kubeadmcontrolplanes.yaml

@@ -1,4 +1,30 @@
 
+---
+apiVersion: admissionregistration.k8s.io/v1beta1
+kind: MutatingWebhookConfiguration
+metadata:
+  creationTimestamp: null
+  name: mutating-webhook-configuration
+webhooks:
+- clientConfig:
+    caBundle: Cg==
+    service:
+      name: webhook-service
+      namespace: system
+      path: /mutate-cluster-x-k8s-io-v1alpha3-kubeadmcontrolplane
+  failurePolicy: Fail
+  name: default.kubeadmcontrolplane.cluster.x-k8s.io
+  rules:
+  - apiGroups:
+    - cluster.x-k8s.io
+    apiVersions:
+    - v1alpha3
+    operations:
+    - CREATE
+    - UPDATE
+    resources:
+    - kubeadmcontrolplane
+
 ---
 apiVersion: admissionregistration.k8s.io/v1beta1
 kind: ValidatingWebhookConfiguration