objects: add configuration upgrade system

Add a very basic configuration update mechanism to objects.
An object can define the UpgradeConfig method, which will be called at
init time, and use it to fill in missing defaults in the configuration.
In the future we may want to make it more complex, for example adding
the config version, but for now a basic solution will do.

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>
(cherry picked from commit 560428be)

lib/objects.py

@@ -58,6 +58,7 @@ class ConfigObject(object):
   def __init__(self, **kwargs):
     for k, v in kwargs.iteritems():
       setattr(self, k, v)
+    self.UpgradeConfig()
 
   def __getattr__(self, name):
     if name not in self.__slots__:
@@ -165,6 +166,15 @@ class ConfigObject(object):
     """Implement __repr__ for ConfigObjects."""
     return repr(self.ToDict())
 
+  def UpgradeConfig(self):
+    """Fill defaults for missing configuration values.
+
+    This method will be called at object init time, and its implementation will
+    be object dependent.
+
+    """
+    pass
+
 
 class TaggableObject(ConfigObject):
   """An generic class supporting tags.