make stack_delete_after decorator tolerate deleted stacks

Make the stack_delete_after decorator try/except so it doesn't cause a
test failure if the test has already done a delete during the test

Change-Id: I4a5e096478ece0de02cc910366a5d361d5ee45e0

heat/tests/utils.py

@@ -74,6 +74,9 @@ def stack_delete_after(test_fn):
         try:
             test_fn(test_cls)
         finally:
-            test_cls.stack.delete()
+            try:
+                test_cls.stack.delete()
+            except AttributeError:
+                print "Could not delete stack (already deleted?)"
         print "Exited", test_fn.__name__
     return wrapped_test