Distinguish viable vs active siblings

Now viable siblings -- siblings where siblings job could run on
(e.g. after requested cluster and cluster features applied) and active
siblings are distinguished. The remote sibling jobs only need to know
about the viable siblings and not the actual siblings. This simplies
things a little bit by not having to update the remote sibling jobs when
the active siblings change (e.g. cluster rejects the submission), only
when the viable siblings are changed (scontrol update clusterfeatures).