Check for available disk space and warn if not sufficient when adding node to cluster

From customer:
Check for available disk space before starting a rebalance. We've occasionally been bitten by forgetting to set the data location correctly (or a previously filed bug where that setting doesn't stick correctly). Having a warning from the webconsole if the data location doesn't appear to have enough space for the projected data storage would be helpful (I realise we don't know exactly how big the data files would be, but an educated guess would probably suffice) Running out of disk space can cause a rebalance to fail, and in some cases cause the node to fail due to filling a system partition, so it would be nice to introduce a warning about lack of disk space before the rebalance starts.

Would probably be best to just check that there is at least the same amount of disk space (in the data directory) as available on other nodes. This should probably display in the UI and on the CLI, but be able to be overridden via the REST API directly.