Use more specific asserts in tests

Instead of assertTrue and assertFalse use more specific asserts.
They are compatible with Python 2.7[1] and 3.4[2]

[1]: https://docs.python.org/2.7/library/unittest.html
[2]: https://docs.python.org/3.4/library/unittest.html

Change-Id: Ifee66714db561fb329911395b2cfdd90c689b609
diff --git a/functional/test_resource_chain.py b/functional/test_resource_chain.py
index 1086e74..2898ebe 100644
--- a/functional/test_resource_chain.py
+++ b/functional/test_resource_chain.py
@@ -58,7 +58,7 @@
 
         # Verify
         stack = self.client.stacks.get(stack_id)
-        self.assertTrue(stack is not None)
+        self.assertIsNotNone(stack)
 
         # Top-level resource for chain
         expected = {'my-chain': 'OS::Heat::ResourceChain'}
@@ -74,15 +74,15 @@
 
         # Outputs
         resource_ids = self._stack_output(stack, 'resource-ids')
-        self.assertTrue(resource_ids is not None)
+        self.assertIsNotNone(resource_ids)
         self.assertEqual(2, len(resource_ids))
 
         resource_value = self._stack_output(stack, 'resource-0-value')
-        self.assertTrue(resource_value is not None)
+        self.assertIsNotNone(resource_value)
         self.assertEqual(8, len(resource_value))  # from parameter
 
         resource_attrs = self._stack_output(stack, 'all-resource-attrs')
-        self.assertTrue(resource_attrs is not None)
+        self.assertIsNotNone(resource_attrs)
         self.assertIsInstance(resource_attrs, dict)
         self.assertEqual(2, len(resource_attrs))
         self.assertEqual(8, len(resource_attrs['0']))