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/pod-nginx.yaml

@@ -5,6 +5,6 @@ metadata:
 spec:
   containers:
   - name: nginx
-    image: nginx
+    image: nginx:1.7.9
     ports:
     - containerPort: 80