Cache zuul status.json for one second.

Serve previously rendered zuul status.json if it is was formatted less
than one second prior. This caches the status.json reducing expensive
calls to format the status.json.

The cache ttl is configurable using the zuul.status_expiry configuration
option in the zuul config.

Change-Id: Ib3fcd75f30bc4d4ef5f09d2adfd566d10c58a991
3 files changed