Support for launching YARN clusters using spark-ec2

This change adds support for launching YARN clusters using spark-ec2 scripts as described in SPARK-3674. 
Still TODO: Test if changes are backwards compatible (i.e. we can launch hadoop 1, hadoop 2 mr1 clusters with these config changes).

cc @pwendell @andrewor14 for review

[I'd especially like somebody who knows YARN better to review the config files like yarn-site.xml to make sure we have all the right options.]