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/pod2.yaml

@@ -7,7 +7,7 @@ desiredState:
     id: storage
     containers:
       - name: redis
-        image: dockerfile/redis
+        image: redis
         volumeMounts:
             # name must match the volume name below
           - name: redis-persistent-storage