[COOK-4434] writing a timestamp: not convergent

[Ticket COOK-4434](https://tickets.opscode.com/browse/COOK-4434)

Every time Chef runs, it will update the template. This is totally idempotent, however it is not convergent. In a world where report handlers will track the number of resources updated in a Chef run, and potentially the timestamp of that run (depending on implmentation), having the last run's time stamp in the motd is less useful.