Catch SIGTERM and SIGINT to clean up processes.

Addresses #144 

Previously we relied on `atexit` to call a cleanup function, but the `atexit` handlers don't run if the process is killed with SIGTERM. We send SIGTERM to MO when running `mongo-orchestration stop`, however.
Cleaning up processes involves iterating over this `_storage` variable, which will raise a TypeError if None.