Do not assign blocks to a datanode with < x mb free

We're running a smallish cluster with very different machines, some with only 60 gb harddrives
This creates a problem when inserting files into the dfs, these machines run out of space quickly and then they cannot run any map reduce operations

A solution would be to not assign any new blocks once the space is below a certain user configurable threshold
This free space could then be used by the map reduce operations instead (if that's on the same disk)