Handle SIGTERM and SIGINT with condor_off, allow for off strategies

[rynge]

SIGTERM and SIGINT will be trapped and mapped to the appropriate condor_off command.

You can choose strategy by setting the SHUTDOWN_STRATEGY environment variable to one of peaceful, graceful or fast.

graceful provides 24 hours for jobs to finish.

fast is default for now, matching the current behavior. We could have policy discussion on how for example the RPM should shutdown. For K8s deployments, graceful with something like this works well:

spec:
  terminationGracePeriodSeconds: 86400
  containers:
  ...
    env:
      - name: SHUTDOWN_STRATEGY
        value: "graceful"