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.
diff --git a/tempest/cmd/verify_tempest_config.py b/tempest/cmd/verify_tempest_config.py
index 0a1881c..6b81e0d 100644
--- a/tempest/cmd/verify_tempest_config.py
+++ b/tempest/cmd/verify_tempest_config.py
@@ -167,15 +167,13 @@
 def get_extension_client(os, service):
     extensions_client = {
         'nova': os.extensions_client,
-        'cinder': os.volumes_extension_client,
         'neutron': os.network_extensions_client,
         'swift': os.capabilities_client,
+        # NOTE: Cinder v3 API is current and v2 and v1 are deprecated.
+        # V3 extension API is the same as v2, so we reuse the v2 client
+        # for v3 API also.
+        'cinder': os.volumes_v2_extension_client,
     }
-    # NOTE (e0ne): Use Cinder API v2 by default because v1 is deprecated
-    if CONF.volume_feature_enabled.api_v2:
-        extensions_client['cinder'] = os.volumes_v2_extension_client
-    else:
-        extensions_client['cinder'] = os.volumes_extension_client
 
     if service not in extensions_client:
         print('No tempest extensions client for %s' % service)