Replace references to Replication Controllers with Deployments in docs/user-guide/walkthrough.

Also:
* Cleanup pasted yaml and instead embed file
* Clarify Service test sections by including busybox exec commands as part of the preformatted section

docs/user-guide/walkthrough/deployment-update.yaml

@@ -0,0 +1,16 @@
+apiVersion: extensions/v1beta1
+kind: Deployment
+metadata:
+  name: nginx-deployment
+spec:
+  replicas: 2
+  template:
+    metadata:
+      labels:
+        app: nginx
+    spec:
+      containers:
+      - name: nginx
+        image: nginx:1.8 # Update the version of nginx from 1.7.9 to 1.8
+        ports:
+        - containerPort: 80