Add support for declaring named volumes in compose files

* Bump default API version to 1.21 (required for named volume management)
* Introduce new, versioned compose file format while maintaining support
  for current (legacy) format
* Test updates to reflect changes made to the internal API

Signed-off-by: Joffrey F <joffrey@docker.com>

tests/integration/service_test.py

@@ -163,6 +163,7 @@ class ServiceTest(DockerClientTestCase):
 
         # Match the last component ("host-path"), because boot2docker symlinks /tmp
         actual_host_path = container.get_mount(container_path)['Source']
+
         self.assertTrue(path.basename(actual_host_path) == path.basename(host_path),
                         msg=("Last component differs: %s, %s" % (actual_host_path, host_path)))