Stop using dockerfile/* images

As per
http://blog.docker.com/2015/03/updates-available-to-popular-repos-update-your-images/
docker has stopped answering dockerfile/redis and dockerfile/nginx.  Fix all
users in our tree.  Sadly this means a lot of published examples are now broken.

examples/walkthrough/k8s201.md

@@ -41,7 +41,7 @@ desiredState:
         id: nginx
         containers:
           - name: nginx
-            image: dockerfile/nginx
+            image: nginx
             ports:
               - containerPort: 80
     # Important: these labels need to match the selector above
@@ -135,7 +135,7 @@ desiredState:
     id: php
     containers:
       - name: nginx
-        image: dockerfile/nginx
+        image: nginx
         ports:
           - containerPort: 80
         # defines the health checking