hadoop: tune max {map,reduce} tasks to num CPUs available

This pull request is in response to issue https://github.com/jtriley/StarCluster/issues/115

Hadoop defaults to 2 maps and 1 reduce per node/machine, and starcluster's hadoop plugin uses the default configs.  For large AWS instance types, this configuration leaves much CPU capacity unutilized.  This change creates a custom mapred-site.xml file for each node that configures the mapred.tasktracker.{map,reduce}.tasks.maximum parameters based upon the node's CPU count.  In particular, the change employs a simple heuristic (similar to the one used in EMR's hadoop configs) that assigns 1 map per CPU and ~1/3 reduce per CPU.  The params are included as kwargs to the plugin's constructor, so the user can override this in the plugin's config.

I have manually tested this change using the following (key) starcluster config settings:

[cluster microdumbo]
NODE_INSTANCE_TYPE = c1.xlarge
CLUSTER_SIZE = 2
PLUGINS = hadoop
NODE_IMAGE_ID = ami-765b3e1f

and ran this procedure on the master node:

$ wget http://norvig.com/big.txt
$ pip install mrjob
$ export HADOOP_HOME=/usr/lib/hadoop
$ python /usr/local/lib/python2.7/dist-packages/mrjob/examples/mr_word_freq_count.py -r hadoop big.txt big.txt big.txt big.txt big.txt big.txt big.txt big.txt big.txt big.txt big.txt big.txt big.txt big.txt big.txt big.txt big.txt big.txt big.txt big.txt

The job tracker showed 2 nodes, 16 maps, and 4 reduces available, and the job ran up to 16 map tasks in parallel, as desired.  The output looked correct.