Provide usable __repr__ for serializable objects

These objects are frequently logged or compared in unit tests.
It's very helpful to be able to inspect their content.

Change-Id: Ib725dcd5f54f4492205f95974d887b8b42c74039

ironic_python_agent/encoding.py

@@ -24,6 +24,11 @@ class Serializable(object):
         """Turn this object into a dict."""
         return dict((f, getattr(self, f)) for f in self.serializable_fields)
 
+    def __repr__(self):
+        fields = " ".join(f"{f}=" + repr(getattr(self, f))
+                          for f in self.serializable_fields)
+        return f"<{self.__class__.__name__} {fields}>"
+
 
 class SerializableComparable(Serializable):
     """A Serializable class which supports some comparison operators

ironic_python_agent/tests/unit/test_encoding.py

@@ -43,6 +43,8 @@ class TestSerializable(base.IronicAgentTest):
         expected = {'jack': 'hello', 'jill': 'world'}
         obj = SerializableTesting('hello', 'world')
         self.assertEqual(expected, obj.serialize())
+        self.assertEqual(
+            "<SerializableTesting jack='hello' jill='world'>", repr(obj))
 
 
 class TestSerializableComparable(base.IronicAgentTest):