Add a 'nodegroups' slot to ConfigData

Also:
  - reformat the __slots__ declaration of ConfigData
  - call UpgradeConfig on each node groups to handle future upgrades
  - add nodegroups special case in {To,From}Dict
  - add nodegroups to _AllUUIDObjects

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>