use multiple store engines, backward compatible

Change-Id: I8e650c15a359eab176b1a1d7d16e8014bd0826e1
diff --git a/README.rst b/README.rst
index 7fa9cd9..50af4ff 100644
--- a/README.rst
+++ b/README.rst
@@ -134,7 +134,7 @@
         enabled: true
         version: juno
         storage:
-          engine: rbd
+          engine: rbd,http
           user: glance
           pool: images
           chunk_size: 8