config.py, objects.py: more node group support
In particular:
- add AddNodeGroup and RemoveNodeGroup in config.py
- refactor _UpgradeConfig to make use of _UnlockedAddNodeGroup
- add support for objects.NodeGroup in ConfigWriter.Update; add missing
"serial_no" attribute to NodeGroup objects
- update NodeGroup.UpgradeConfig to set mtime and serial_no if missing
(the refactored code in _UpgradeConfig was not settting mtime nor ctime)
Signed-off-by: Adeodato Simo <dato@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>