Use addClassResourceCleanup for cleanup in test_regions

This is to use addClassResourceCleanup in test_regions.py
so the resource_cleanup can be removed.

Change-Id: I9fb0a18c5ff78d9cf114888b3197a50971032ad0
diff --git a/tempest/api/identity/admin/v3/test_regions.py b/tempest/api/identity/admin/v3/test_regions.py
index d00e408..f22a528 100644
--- a/tempest/api/identity/admin/v3/test_regions.py
+++ b/tempest/api/identity/admin/v3/test_regions.py
@@ -34,14 +34,10 @@
             r_description = data_utils.rand_name('description')
             region = cls.client.create_region(
                 description=r_description)['region']
+            cls.addClassResourceCleanup(
+                cls.client.delete_region, region['id'])
             cls.setup_regions.append(region)
 
-    @classmethod
-    def resource_cleanup(cls):
-        for r in cls.setup_regions:
-            cls.client.delete_region(r['id'])
-        super(RegionsTestJSON, cls).resource_cleanup()
-
     @decorators.idempotent_id('56186092-82e4-43f2-b954-91013218ba42')
     def test_create_update_get_delete_region(self):
         # Create region