Modify use of assertTrue(A in B)

Developers should use assertIn(A, B) instead of assertTrue(A in B ).

TrivialFix

Change-Id: I6f2b33e73ff3ebf28a681021d58c9b2485a016fd
diff --git a/functional/test_env_merge.py b/functional/test_env_merge.py
index 5e222b8..819770a 100644
--- a/functional/test_env_merge.py
+++ b/functional/test_env_merge.py
@@ -92,4 +92,4 @@
         # by checking to see that it has a value
         r3b = self.client.resources.get(stack_id, 'r3b')
         r3b_attrs = r3b.attributes
-        self.assertTrue('value' in r3b_attrs)
+        self.assertIn('value', r3b_attrs)