implement cluster-wide setting for bucket's watermarks (was: Added to cluster nodes should take into account custom watermark settings)

1. Create cluster, 2 nodes, 1 bucket
2. Change default values of low/high watermarks
3. Add 3rd node to cluster, rebalance it.

That new node will still use default thresholds (75%/85%). Apparently it doesn't make sense from end-user perspective.