Add CI for images/kube-webhook-certgen (#7717)

As a follow up to PR #7641, this commit adds some basic e2e tests for
kube-webhook-certgen image.

Signed-off-by: Mateusz Gozdek <mgozdek@microsoft.com>

images/kube-webhook-certgen/hack/e2e.yaml

@@ -0,0 +1,57 @@
+apiVersion: v1
+kind: Namespace
+metadata:
+  name: test
+---
+apiVersion: admissionregistration.k8s.io/v1
+kind: ValidatingWebhookConfiguration
+metadata:
+  name: "pod-policy.example.com"
+webhooks:
+- name: "pod-policy.example.com"
+  rules:
+  - apiGroups:   [""]
+    apiVersions: ["v1"]
+    operations:  ["CREATE"]
+    resources:   ["pods"]
+    scope:       "Namespaced"
+  clientConfig:
+    service:
+      namespace: "example-namespace"
+      name: "example-service"
+  admissionReviewVersions: ["v1", "v1beta1"]
+  sideEffects: None
+  timeoutSeconds: 5
+---
+apiVersion: admissionregistration.k8s.io/v1
+kind: MutatingWebhookConfiguration
+metadata:
+  name: "pod-policy.example.com"
+webhooks:
+- name: "pod-policy.example.com"
+  rules:
+  - apiGroups:   [""]
+    apiVersions: ["v1"]
+    operations:  ["CREATE"]
+    resources:   ["pods"]
+    scope:       "Namespaced"
+  clientConfig:
+    service:
+      namespace: "example-namespace"
+      name: "example-service"
+  admissionReviewVersions: ["v1", "v1beta1"]
+  sideEffects: None
+  timeoutSeconds: 5
+---
+apiVersion: apiregistration.k8s.io/v1
+kind: APIService
+metadata:
+  name: v1alpha1.pod-policy.example.com
+spec:
+  service:
+    name: "example-namespace"
+    namespace: "example-service"
+  group: pod-policy.example.com
+  version: v1alpha1
+  groupPriorityMinimum: 100
+  versionPriority: 100