Reuse v2 extension client for cinder v3

Now(Pike) Cinder versions are like
 - V1: Deprecated
 - V2: Deprecated
 - V3: Current

So users should be able to disable cinder v2 API on Tempest config.
This patch makes verify_tempest_config reuse v2 extension client for
v3 also. In addition, this patch removes v1 extension client usage
on verify_tempest_config because Tempest doesn't contain v1 tests
at all and we don't need to verify the v1 extensions at all.

Change-Id: I3ef5a82a70fa9e4b862f50a85f93351888ed6bcd
diff --git a/releasenotes/notes/use-cinder-v3-client-for-verify_tempest_config-2bf3d817b0070064.yaml b/releasenotes/notes/use-cinder-v3-client-for-verify_tempest_config-2bf3d817b0070064.yaml
new file mode 100644
index 0000000..1c8fa77
--- /dev/null
+++ b/releasenotes/notes/use-cinder-v3-client-for-verify_tempest_config-2bf3d817b0070064.yaml
@@ -0,0 +1,6 @@
+---
+upgrade:
+  - verify_tempest_config command starts using extension_client of
+    cinder v2 API only, because cinder v3 API is current and v2 and
+    v1 are deprecated and v3 extension API is the same as v2. Then
+    we can reuse the v2 client for v3 API also.