Replace assertItemsEqual with assertCountEqual

assertItemsEqual was removed from Python's unittest.TestCase in
Python 3.3 [1][2]. We have been able to use them since then, because
testtools required unittest2, which still included it. With testtools
removing Python 2.7 support [3][4], we will lose support for
assertItemsEqual, so we should switch to use assertCountEqual.

[1] - https://bugs.python.org/issue17866
[2] - https://hg.python.org/cpython/rev/d9921cb6e3cd
[3] - testing-cabal/testtools#286
[4] - testing-cabal/testtools#277

Change-Id: Id354d9e30b6fa6f4b72116a659d16a2fb8030c87

octavia/tests/functional/api/v2/test_listener.py

@@ -2575,7 +2575,7 @@ class TestListener(base.BaseAPITest):
         self.put(listener_path,
                  self._build_body({'sni_container_refs': [sni_id1, sni_id2]}))
         get_listener = self.get(listener_path).json['listener']
-        self.assertItemsEqual([sni_id1, sni_id2],
+        self.assertCountEqual([sni_id1, sni_id2],
                               get_listener.get('sni_container_refs'))
 
     # TODO(johnsom) Fix this when there is a noop certificate manager