roles/nginx: Add fastcgi caching

Bypasses caching for logged in users (right now only for sessions
where the "wordpress_logged_in" cookie is set. Doubles the trans-
actions per second as measured by siege:

    $ siege -d1 -t1M -c50 https://mjanja.ch

Signed-off-by: Alan Orth <alan.orth@gmail.com>

roles/nginx/files/nginx.conf

@@ -37,5 +37,6 @@ http {
 
     client_max_body_size 12m;
 
+    include fastcgi_cache;
     include /etc/nginx/conf.d/*.conf;
 }