Rework the data loader pipelines to read groups

This (invasive) patch changes all the loader pipelines to read the node
groups data from the cluster, via the various backends. It is invasive
as it needs coordinated changes across all the loaders.

Note that the new group data is not used, just returned.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Balazs Lecz <leczb@google.com>