web: add Cache-Control to static files

This change add the Cache-Control header to static files' response.

Change-Id: Ibdf1c35bad378507162d807cf5acdf13fc3fab88
diff --git a/etc/zuul.conf-sample b/etc/zuul.conf-sample
index 76494ad..f0e1765 100644
--- a/etc/zuul.conf-sample
+++ b/etc/zuul.conf-sample
@@ -37,6 +37,7 @@
 [web]
 listen_address=127.0.0.1
 port=9000
+static_cache_expiry=0
 
 [webapp]
 listen_address=0.0.0.0