Provide "All Pass" status on /heartbeat

ISO's E2E checks of manage.jujucharms.com currently monitor http://manage.jujucharms.com/api/2/charms/interesting. I found out about http://manage.jujucharms.com/heartbeat, which is a better human-readable high-level check, but provides nothing Nagios can latch on to.

I'd like to propose adding a summary line item, where if all items pass, it should say e.g. "All Pass". We can then throw Nagios at that page and check for the presence or absence of that phrase.