Add tearDownClass to base swift test class

This commit fixes an oversight which was preventing isolated
credential cleanup on the swift tests.

Change-Id: Ia54297351d5889aa4a19b0fc452b45838392724a
diff --git a/tempest/api/object_storage/base.py b/tempest/api/object_storage/base.py
index e6e8d17..1d16b2f 100644
--- a/tempest/api/object_storage/base.py
+++ b/tempest/api/object_storage/base.py
@@ -73,6 +73,11 @@
         cls.data = DataGenerator(cls.identity_admin_client)
 
     @classmethod
+    def tearDownClass(cls):
+        cls.isolated_creds.clear_isolated_creds()
+        super(BaseObjectTest, cls).tearDownClass()
+
+    @classmethod
     def _assign_member_role(cls):
         primary_user = cls.isolated_creds.get_primary_user()
         alt_user = cls.isolated_creds.get_alt_user()