Update supported version doc tempest

This patch updates about the new supported release Ussuri.

Change-Id: Ib17602dfe0eb1707acfe220ab0c230cf781cd367
diff --git a/doc/source/supported_version.rst b/doc/source/supported_version.rst
index 62faa1f..388b4cd 100644
--- a/doc/source/supported_version.rst
+++ b/doc/source/supported_version.rst
@@ -9,6 +9,7 @@
 
 Tempest master supports the below OpenStack Releases:
 
+* Ussuri
 * Train
 * Stein