Modify use of assertTrue(A in B)

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

TrivialFix

Change-Id: I54158400476fdd5cfd40653408b33fb0f97b60c0
diff --git a/tempest/api/identity/admin/v2/test_users.py b/tempest/api/identity/admin/v2/test_users.py
index 8e63498..4a4b51a 100644
--- a/tempest/api/identity/admin/v2/test_users.py
+++ b/tempest/api/identity/admin/v2/test_users.py
@@ -234,4 +234,4 @@
         # Validate the updated password through getting a token.
         body = self.token_client.auth(user['name'], new_pass,
                                       tenant['name'])
-        self.assertTrue('id' in body['token'])
+        self.assertIn('id', body['token'])