Add ability to specify s3 bootstrap script to spark-ec2

One of the nice features of using EMR to start spark clusters is that you can specify bootstrap actions to run on each node. This is useful for installing custom libraries, etc. However, you pay a per-node/per-hour premium for using EMR (https://aws.amazon.com/elasticmapreduce/pricing/).

This pull request adds the --bootstrap-script flag to the spark-ec2 script. This flag specifies an s3 path to a shell script. The script is downloaded and run on each node during the setup process. The flag can be specified multiple times for multiple bootstrap actions.