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/v1beta3/README.md

@@ -40,7 +40,7 @@ metadata:
 spec:
   containers:
   - name: nginx
-    image: dockerfile/nginx
+    image: nginx
     volumeMounts:
     - mountPath: /srv/www
       name: www-data

examples/walkthrough/v1beta3/pod-with-http-healthcheck.yaml

@@ -5,7 +5,7 @@ metadata:
 spec:
   containers:
     - name: nginx
-      image: dockerfile/nginx
+      image: nginx
       # defines the health checking
       livenessProbe:
         # an http probe

examples/walkthrough/v1beta3/pod1.yaml

@@ -5,4 +5,4 @@ metadata:
 spec:
   containers:
     - name: nginx
-      image: dockerfile/nginx
+      image: nginx

examples/walkthrough/v1beta3/pod2.yaml

@@ -5,7 +5,7 @@ metadata:
 spec:
   containers:
     - name: redis
-      image: dockerfile/redis
+      image: redis
       volumeMounts:
           # name must match the volume name below
         - name: redis-persistent-storage

examples/walkthrough/v1beta3/replication-controller.yaml

@@ -19,6 +19,6 @@ spec:
     spec:
       containers:
         - name: nginx
-          image: dockerfile/nginx
+          image: nginx
             ports:
             - containerPort: 80