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/persistent-volumes/simpletest/pod.yaml

@@ -4,7 +4,7 @@ metadata:
   name: mypod
 spec:
   containers:
-    - image: dockerfile/nginx
+    - image: nginx
       name: myfrontend
       volumeMounts:
       - mountPath: "/var/www/html"