Webapp now emits Cache-Control header

Running Zuul behind a shared proxy, the status pages ended up being
cached which is slightly annoying. Add to the Webapp response a
Cache-Control header with ages set to Zuul configuration parameter
`status_expiry`.

Change-Id: Id3f8ebe271de5daf7964a5fef91444fd37d34b18
Merge https://review.openstack.org/#/c/132073/2

a29f7f3 Webapp now emits Cache-Control header

Zuul status page is frozen, status.json is cached by misc Varnish
Bug: 72766