KeyError when tearDownClass called from setUpClass

The setUpclass full chain detection realys on calling at leat
 one test_method, but this is not happening when the setUpClass
failing.

Partially Fixing Bug: 1218279

Change-Id: I679d328591b10c8f0a8f3cb66ba250cdff2ecf05
diff --git a/tempest/test.py b/tempest/test.py
index 8b3cfa6..63cdbdf 100644
--- a/tempest/test.py
+++ b/tempest/test.py
@@ -141,7 +141,7 @@
 
     @classmethod
     def tearDownClass(cls):
-        at_exit_set.remove(cls)
+        at_exit_set.discard(cls)
         if hasattr(super(BaseTestCase, cls), 'tearDownClass'):
             super(BaseTestCase, cls).tearDownClass()