We weren't catching SIGTERM and cancelling our context, so the go
runtime would have us exit with a non-zero code when the signal was
received. Let's catch it and try to exit gracefully.

i.e. before:

    (...)
    I0508 11:08:00.426917 1787442 log_monitor.go:237] Initialize condition generated: []
    ^C
    $ echo $?
    130

and now:

    (...)
    I0508 11:07:42.150562 1787146 log_monitor.go:237] Initialize condition generated: []
    ^C
    I0508 11:07:43.372093 1787146 log_monitor.go:123] Stop log monitor /tmp/npd-test.json
    I0508 11:07:43.652876 1787146 log_watcher.go:104] Stop watching filelog
    I0508 11:07:43.652923 1787146 log_monitor.go:144] Log monitor stopped: /tmp/npd-test.json
    $ echo $?
    0