Add tests for manifest lists and fix some issues encountered while testing

image/docker/interfaces.py

@@ -4,6 +4,10 @@ from six import add_metaclass
 @add_metaclass(ABCMeta)
 class ManifestInterface(object):
   """ Defines the interface for the various manifests types supported. """
+  @abstractproperty
+  def schema_version(self):
+    """ The version of the schema, or None for lists. """
+
   @abstractproperty
   def digest(self):
     """ The digest of the manifest, including type prefix. """