webapp: set cache-control headers to prevent caching

Whenever Zuul daemon is behind a proxy cache (such as Varnish), it would
cache the status page which is not very convenient.  This patch adds
'Cache-Control: no-cache' which would prevent webcache from caching the
status page ensuring users have the latest status.

Change-Id: I5c2b4f680c8e040c1ca1b1a4d797cc01a124677a