Fix ResetStateOnErrorTest.test_success

The new test ResetStateOnErrorTest.test_success relied on a class level
variable to do assertions, thus being sensitive to test order. This
moves the culprit variable to the instance level.

Change-Id: I09705903939530be0983aab830d27d80634fe23f
Closes-Bug: #1517359

heat/tests/test_stack.py

@@ -2495,7 +2495,8 @@ class ResetStateOnErrorTest(common.HeatTestCase):
         action = 'something'
         state = COMPLETE
 
-        set_state = mock.MagicMock()
+        def __init__(self):
+            self.set_state = mock.MagicMock()
 
         @stack.reset_state_on_error
         def raise_exception(self):