Accommodate v2 and v3 auth for integration tests

devstack has moved default Keystone API version to v3[1].

[1] https://github.com/openstack-dev/devstack/commit/f4ce44bf3fbf06e53c2ae3ec6aa4996831cf4605

Though the above patch has been reverted, this would help
if devstack removes v2 support in the future.

Change-Id: I393750d00b3712a015e48a3cf38ab5f95bb61dae
Closes-Bug: #1539692
diff --git a/functional/test_preview.py b/functional/test_preview.py
index b927a25..5846523 100644
--- a/functional/test_preview.py
+++ b/functional/test_preview.py
@@ -43,7 +43,7 @@
     def setUp(self):
         super(StackPreviewTest, self).setUp()
         self.client = self.orchestration_client
-        self.project_id = self.identity_client.auth_ref.project_id
+        self.project_id = self.identity_client.project_id
 
     def _assert_resource(self, res, stack_name):
         self.assertEqual(stack_name, res['stack_name'])