Adapt functional tests to python3

A test looking for a regex in an argparse error message is failing when ran
on python3. Since this is now the default python runtime for devstack-based
jobs, this updates the message.

Change-Id: Id3372998715f13f7641d11a4a1d9d16629acfd18
Story: 2006896
Task: 37530

Author: lukapeschke

cloudkittyclient/tests/functional/v2/test_dataframes.py

@@ -139,7 +139,7 @@ def tearDown(self):
     def test_dataframes_add_with_no_args(self):
         self.assertRaisesRegexp(
             RuntimeError,
-            'error: too few arguments',
+            'error: the following arguments are required: datafile',
             self.runner,
             'dataframes add',
             fmt='',