add apply test for retainKeys

hack/testdata/retainKeys/deployment/deployment-after.yaml

@@ -0,0 +1,22 @@
+apiVersion: extensions/v1beta1
+kind: Deployment
+metadata:
+  name: test-deployment-retainkeys
+spec:
+  strategy:
+    type: Recreate
+  replicas: 1
+  template:
+    metadata:
+      labels:
+        app: nginx
+    spec:
+      containers:
+      - name: nginx
+        image: nginx
+        ports:
+        - containerPort: 80
+      volumes:
+      - name: test-volume
+        hostPath:
+          path: /data

hack/testdata/retainKeys/deployment/deployment-before.yaml

@@ -0,0 +1,18 @@
+apiVersion: extensions/v1beta1
+kind: Deployment
+metadata:
+  name: test-deployment-retainkeys
+spec:
+  replicas: 1
+  template:
+    metadata:
+      labels:
+        app: nginx
+    spec:
+      containers:
+      - name: nginx
+        image: nginx
+        ports:
+        - containerPort: 80
+      volumes:
+      - name: test-volume