Enable stopping and starting a spot cluster

Clusters launched using `--spot-price` contain an on-demand master and spot slaves. Because EC2 does not support stopping spot instances, the spark-ec2 script previously could only destroy such clusters.

This pull request makes it possible to stop and restart a spot cluster.
* The `stop` command works as expected for a spot cluster: the master is stopped and the slaves are terminated.
* To restart a stopped spot cluster, the user must invoke `launch --use-existing-master`. This launches fresh spot slaves but restarts the existing master.