Improve cleanup of ssh processes in bosco_cluster.
Improve cleanup of ssh processes in bosco_cluster.

bosco_cluster would often leave ssh processes running when exiting early
due to an error. Now, it cleans up on shell exit when necessary.
HTCONDOR-21

htcondor/htcondor,htcondor/htcondor,htcondor/htcondor,htcondor/htcondor,htcondor/htcondor,htcondor/htcondor,htcondor/htcondor,htcondor/htcondor