Make working_dir consistent in the config package.

- make it a positional arg, since it's required
- make it the first argument for all functions that require it
- remove an unnecessary one-line function that was only called in one place

Signed-off-by: Daniel Nephin <dnephin@docker.com>

tests/unit/config/config_test.py

@@ -577,7 +577,7 @@ class VolumeConfigTest(unittest.TestCase):
 
     def test_volume_path_with_non_ascii_directory(self):
         volume = u'/Füü/data:/data'
-        container_path = config.resolve_volume_path(volume, ".", "test")
+        container_path = config.resolve_volume_path(".", volume, "test")
         self.assertEqual(container_path, volume)