Add feature to allow CDDBaseline to kill spawned processes

[timmattison]

When operating outside of the Greengrass container we should provide users with a way to indicate that they want their spawned processes to be cleaned up when the core restarts or is redeployed.

This could be done by adding a special environment variable to each process it spawns. At shutdown time we would check all of the PID we launched (or check proc to find all running PIDs) and see if they are still running and if they still contain the special variable. If they do they should be terminated.

The user should also have the option to not clean these processes up if they need them to run longer than the core's lifetime.