Fix gnt-backup list -o node via confd

Currently, the 'node' field is declared as a simple config field, so
when only selecting this fields, the runtime gathering is no longer
run and it's presumed that all nodes have a backup. So the output is
not truthful (instead of just listing the nodes with at least one
export, all are listed).

By simply making it a runtime field, filtering works properly and we
get the same output as the masterd code.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Helga Velroyen <helgav@google.com>