Disable Cinder v1 API support by default

Cinder API v1 will be removed Mitaka so we don't need to setup it be
default.

To enable Cinder API v1 you need to set CINDER_ENABLE_V1_API=True in
your Devstack config.

Related-Bug: #1467589

Depends-On: I6916eb3e4b7c85f37be8b365b11ca8b48f88177c
Change-Id: I0754e357433cfcd9fde7e937a4a1b440580b6289
diff --git a/doc/source/configuration.rst b/doc/source/configuration.rst
index 983f5c0..3bd246d 100644
--- a/doc/source/configuration.rst
+++ b/doc/source/configuration.rst
@@ -587,6 +587,13 @@
         VOLUME_BACKING_FILE_SIZE=10250M
 
 
+Cinder v1 API is depricated and disabled by default. You can enable v1 API by
+setting ``CINDER_ENABLE_V1_API`` to ``True``.
+
+    ::
+        CINDER_ENABLE_V1_API=True
+
+
 Keystone
 ~~~~~~~~