Make encoding.serialize() more programmatical

Introduce `serializable_fields` to express which class attributes
to be serialized.

Get rid of OrderedDict. Just replacing it with regular dict.

Change-Id: I3f7639dab171d3d62e92d0d1bb6d7b071cf963ad

ironic_python_agent/encoding.py

@@ -18,9 +18,11 @@ import uuid
 
 class Serializable(object):
     """Base class for things that can be serialized."""
+    serializable_fields = ()
+
     def serialize(self):
         """Turn this object into a dict."""
-        raise NotImplementedError()
+        return dict((f, getattr(self, f)) for f in self.serializable_fields)
 
 
 class RESTJSONEncoder(json.JSONEncoder):