Cleanup: unneeded client definition, API override

- do not redefine a few variables for volume(admin and not) and backup clients
  when already defined in the base classes;
- define cls.admin_volume_client (same name as in tempest) rather then
  admin_volumes_client in the base class for admin volume tests;
- remove any usage and override of _api_version, which is not really used
  in the plugin, and it was also removed in tempest (see
  I6e05fbe21b29efa128fb59d16bf521d551b00304 )
  Even if unused, _api_version still pointed to the deprecated version 2
  for consistency group tests.

Change-Id: I3164662932a302b87ae2f346c0005bf7d0ab2927
diff --git a/cinder_tempest_plugin/api/volume/base.py b/cinder_tempest_plugin/api/volume/base.py
index 40bfb33..21da190 100644
--- a/cinder_tempest_plugin/api/volume/base.py
+++ b/cinder_tempest_plugin/api/volume/base.py
@@ -165,7 +165,7 @@
 
         cls.admin_volume_types_client = cls.os_admin.volume_types_client_latest
         cls.admin_backups_client = cls.os_admin.backups_client_latest
-        cls.admin_volumes_client = cls.os_admin.volumes_client_latest
+        cls.admin_volume_client = cls.os_admin.volumes_client_latest
 
     @classmethod
     def create_volume_type(cls, name=None, **kwargs):
@@ -188,12 +188,12 @@
         type_id = volume_type['id']
         type_name = volume_type['name']
 
-        volumes = cls.admin_volumes_client.list_volumes(
+        volumes = cls.admin_volume_client.list_volumes(
             detail=True, params={'all_tenants': 1})['volumes']
         for volume in [v for v in volumes if v['volume_type'] == type_name]:
             test_utils.call_and_ignore_notfound_exc(
-                cls.admin_volumes_client.delete_volume, volume['id'])
-            cls.admin_volumes_client.wait_for_resource_deletion(volume['id'])
+                cls.admin_volume_client.delete_volume, volume['id'])
+            cls.admin_volume_client.wait_for_resource_deletion(volume['id'])
 
         test_utils.call_and_ignore_notfound_exc(
             cls.admin_volume_types_client.delete_volume_type, type_id)