Remove "extensions" from volumes_extensions_client

Many Nova APIs are extensions and it is meaningless to contain
the word "extensions" in service client names.
Then this patch removes the word before migrating the service
client to tempest-lib.

Change-Id: Ibd9b6c67c5b9790b6d6f1a76d334ee5d6c98ad5f
diff --git a/tempest/clients.py b/tempest/clients.py
index dd0d145..e5ef812 100644
--- a/tempest/clients.py
+++ b/tempest/clients.py
@@ -81,8 +81,8 @@
 from tempest.services.compute.json.tenant_usages_client import \
     TenantUsagesClient
 from tempest.services.compute.json.versions_client import VersionsClient
-from tempest.services.compute.json.volumes_extensions_client import \
-    VolumesExtensionsClient
+from tempest.services.compute.json.volumes_client import \
+    VolumesClient as ComputeVolumesClient
 from tempest.services.data_processing.v1_1.data_processing_client import \
     DataProcessingClient
 from tempest.services.database.json.flavors_client import \
@@ -334,7 +334,7 @@
             'build_interval': CONF.volume.build_interval,
             'build_timeout': CONF.volume.build_timeout
         })
-        self.volumes_extensions_client = VolumesExtensionsClient(
+        self.volumes_extensions_client = ComputeVolumesClient(
             self.auth_provider, **params_volume)
         self.compute_versions_client = VersionsClient(self.auth_provider,
                                                       **params_volume)