Automatically generate a HTTP secret if none is provided

Log a warning if the registry generates its own secret.

Update configuration doc, and remove the default secret from the
development config file.

Signed-off-by: Aaron Lehmann <aaron.lehmann@docker.com>

cmd/registry/config.yml

@@ -30,7 +30,6 @@ storage:
             enabled: false
 http:
     addr: :5000
-    secret: asecretforlocaldevelopment
     debug:
         addr: localhost:5001
 redis: