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_aws_stack.py b/functional/test_aws_stack.py
index 678ce7e..9649a02 100644
--- a/functional/test_aws_stack.py
+++ b/functional/test_aws_stack.py
@@ -77,7 +77,7 @@
     def setUp(self):
         super(AwsStackTest, self).setUp()
         self.object_container_name = test.rand_name()
-        self.project_id = self.identity_client.auth_ref.project_id
+        self.project_id = self.identity_client.project_id
         self.swift_key = hashlib.sha224(
             str(random.getrandbits(256))).hexdigest()[:32]
         key_header = 'x-container-meta-temp-url-key'