adjust Zuul daemon web access

The Zuul daemon provides an HTTP interface which simply contains two
pages: /status and /status.json.  This patch tweak the proxy
configuration to only proxy /zuul/status and /zuul/status.json leaving
any /zuul/* url to land in the main document root.

Change-Id: Ic36e12e19d328414263ef0e2377b7aa8089e2ce7