Currently, the lab inventory scripts run on the same server as the
Autotest master scheduler.  This no longer makes particularly good
sense.  See especially the comments on the description for bug 881621.

The "sentinel" server has already been set aside for various recurring
tasks.  So, we should just move the lab inventory cron jobs there.
We should move the stable version assignment jobs with it, because it's
in the same cron file.