Switch spark_ec2.py to use the new spark-ec2 scripts. 

This changes spark_ec2.py to use the new cluster initialization scripts from https://github.com/shivaram/spark-ec2. The new scripts preserve existing features, fix a couple of bugs in the standalone setup and also optionally (-g) allow ganglia to be started on the cluster. Users can also fall back to the old script using the --mesos-scripts if required.