Add keystone API versioning support

This patch adds an ability to specify keystone API version.
v3 is default.

Change-Id: I8810672ceef493e7dbd32b755705f4c0ccb30b63
diff --git a/README.rst b/README.rst
index 1a535a7..ece42b8 100644
--- a/README.rst
+++ b/README.rst
@@ -680,6 +680,16 @@
         barbican:
           enabled: true
 
+
+Specify Keystone API version (v3 is default):
+
+.. code-block:: yaml
+
+    cinder:
+      controller:
+        identity:
+          api_version: v2.0
+
 Enhanced logging with logging.conf
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