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/pod1.yaml

@@ -7,4 +7,4 @@ desiredState:
     id: www
     containers:
       - name: nginx
-        image: dockerfile/nginx
+        image: nginx