Improvements to fake topology; changes how racks are assigned

* Assigns only two racks, 'lo' and 'hi'. That's all you should really be using with fake topology
* Better explanation of caveats and motivation
* increased topology.script.number.args so script is used fewer times

Note: the rack assignments have CHANGED. So if you were using this before (I don't think many were) please proceed with eyes open.

When you first add fake topology, you must force Hadoop to migrate blocks to the new policy. This is best done by annealing the replication factor up, then down -- see hadoop_cluster/recipes/fake_topology.rb for instructions.