Pulling EC2 Updates into V2 Branch

This patches the existing pull request into the v2 branch of mesos/spark-ec2. Once it's merged in, I can do some final tests and then we can merge in the mesos/spark half.

One note, the two AMI's in the registry here are private. Tomorrow I'll submit a PR with the full set of public AMI's which we can then test. I've actually written a small test suite which I plan to run in each region to make sure the AMI's have correctly been copied:

https://github.com/pwendell/spark-utils/blob/master/cluster_tests.py

Once this is thoroughly tested I'll ask to merge the one in mesos/spark.