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.

pkg/config/config_test.json

@@ -42,7 +42,7 @@
         "version": "v1beta1",
         "containers": [{
           "name": "master",
-          "image": "dockerfile/redis",
+          "image": "redis",
           "env": [
             {
               "name": "REDIS_PASSWORD",