Use Block Storage API v3 instead of API v2

Block Storage API v2 was deprecated during Pike cycle and is being
removed during Xena cycle, and current v3 API should be used instead.

Change-Id: Ic15f4aff72661ae5a50dbb14840ba0277a578173
diff --git a/heat_tempest_plugin/services/clients.py b/heat_tempest_plugin/services/clients.py
index d6e1dfb..ae4cbe2 100644
--- a/heat_tempest_plugin/services/clients.py
+++ b/heat_tempest_plugin/services/clients.py
@@ -62,7 +62,7 @@
     calling various OpenStack APIs.
     """
 
-    CINDERCLIENT_VERSION = '2'
+    CINDERCLIENT_VERSION = '3'
     HEATCLIENT_VERSION = '1'
     NOVA_API_VERSION = '2.1'
     GNOCCHI_VERSION = '1'