Fix unit test failure with python3.6

Since python3.6 os.path.join can be either bytes or
a str. So mock os.path.join in order for the unit tests to
pass.

Change-Id: I82c793875d888092e5c814727a6c4ad4053e76fb
Signed-off-by: Chuck Short <chucks@redhat.com>

magnum/tests/unit/drivers/test_heat_driver.py

@@ -71,7 +71,8 @@ class TestHeatPoller(base.TestCase):
         self.assertEqual(cluster_status.CREATE_FAILED, cluster.status)
         self.assertEqual('Create failed', cluster.status_reason)
 
-    def test_poll_done(self):
+    @patch('os.path.join')
+    def test_poll_done(self, mock_join):
         mock_heat_stack, cluster, poller = self.setup_poll_test()
 
         mock_heat_stack.stack_status = cluster_status.DELETE_COMPLETE
@@ -124,7 +125,8 @@ class TestHeatPoller(base.TestCase):
         self.assertEqual(cluster_status.ROLLBACK_FAILED, cluster.status)
         self.assertEqual(1, cluster.node_count)
 
-    def test_poll_destroy(self):
+    @patch('os.path.join')
+    def test_poll_destroy(self, mock_join):
         mock_heat_stack, cluster, poller = self.setup_poll_test()
 
         mock_heat_stack.stack_status = cluster_status.DELETE_FAILED