updater should generate static crontab files at install time

[jdreed]

We briefly discussed this a while ago, but didn't implement it due to the "If it ain't broke..." paradigm. It has since broke. Since the desync value is seeded from the hostname, it's going to be constant for each machine (modulo renames), so we should just generate the crontabs at installation time, and then not have to worry about at(1) or running desync each time, or whatever.