Cache status.json for 5 seconds in Apache

There's no need to actually serve to-the-second accurate status.json
content. Cache it in Apache.

Closes-bug: 1326170
Change-Id: If3bc44b781ce4e12ac95d3929242ec0c912f0010

modules/zuul/manifests/init.pp

@@ -278,4 +278,14 @@ class zuul (
       ensure => present,
     }
   }
+  if ! defined(A2mod['cache']) {
+    a2mod { 'cache':
+      ensure => present,
+    }
+  }
+  if ! defined(A2mod['mem_cache']) {
+    a2mod { 'mem_cache':
+      ensure => present,
+    }
+  }
 }

modules/zuul/templates/zuul.vhost.erb

@@ -23,4 +23,11 @@
   AliasMatch ^/p/(.*/objects/pack/pack-[0-9a-f]{40}.(pack|idx))$ /var/lib/zuul/git/$1
   ScriptAlias /p/ /usr/lib/git-core/git-http-backend/
 
+  <IfModule mod_cache.c>
+    CacheDefaultExpire 5
+    <IfModule mod_mem_cache.c>
+      CacheEnable mem /status.json
+    </IfModule>
+  </IfModule>
+
 </VirtualHost>