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>