Development feature: Provide a way to conduct a graceful shutdown of tango or the job manager

[anthony-yip]

Currently, the main way to test changes to Tango is to run the python scripts locally and stop them as necessary with SIGTERM. However, it is possible that such an abnormal termination prevents proper cleanup, so VMs that are created may not be stopped.

A potential fix is to add a SIGTERM handler to Tango that processes the job queue and workers, shutting them down gracefully.