Improvements to fake topology; note: 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.
Hadoop too-small stack size killed tasks on Ubuntu 12
Finally figured out that Hadoop was dying because

```
The stack size specified is too small, Specify at least 160k
Could not create the Java virtual machine.
```

(That's according to oracle java, but not the prior sun java).

This wasn't appearing in the jobtracker's log view; I had to look in var/log/hadoop/userlogs to see the JVM's epitaph. Note that that directory is read-protected, so you will have to sudo to see it.

Error in jobtracker logs looked like

```
2013-01-12 20:30:47,605 INFO org.apache.hadoop.mapred.JobTracker: Adding task (JOB_CLEANUP) 'attempt_201301120857_0005_m_000001_3' to tip task_201301120857_0005_m_000001, for tracker 'tracker_ip-10-113-42-223.ec2.internal:localhost/127.0.0.1:54742'
2013-01-12 20:30:47,605 INFO org.apache.hadoop.mapred.JobTracker: Removing task 'attempt_201301120857_0005_m_000001_2'
2013-01-12 20:30:47,908 INFO org.apache.hadoop.mapred.TaskInProgress: Error from attempt_201301120857_0005_m_000001_3: java.lang.Throwable: Child Error
at org.apache.hadoop.mapred.TaskRunner.run(TaskRunner.java:242)
Caused by: java.io.IOException: Task process exit with nonzero status of 1.
at org.apache.hadoop.mapred.TaskRunner.run(TaskRunner.java:229)
```