Fix tox cover environment on python 3.6

In python 3.6, os.path.join does not accept Mock objects, giving the
following error:

TypeError: expected str, bytes or os.PathLike object, not MagicMock

Since the switch to use ubuntu bionic for the base image for jobs,
python 3.6 is the default for python 3.

Change-Id: I79bec585361a54b8303c130a363c62d9a4b7faae
Story: 2004959
Task: 30035
(cherry picked from commit 14bfce8a72)

kayobe/tests/unit/test_ansible.py

@@ -285,7 +285,8 @@ class TestCase(unittest.TestCase):
                          mock_rmtree):
         parser = argparse.ArgumentParser()
         parsed_args = parser.parse_args([])
-        dump_dir = mock_mkdtemp.return_value
+        dump_dir = "/path/to/dump"
+        mock_mkdtemp.return_value = dump_dir
         mock_listdir.return_value = ["host1.yml", "host2.yml"]
         mock_read.side_effect = [
             {"var1": "value1"},