roles/nginx: Adjust Cache-Control headers

Use "public" with "max-age" instead of Expires, as "max-age" is always
preferred if it's present.  Note: setting "public" doesn't make the
resource "more cacheable", but it is just more explicit.

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

roles/nginx/templates/wordpress.j2

@@ -6,8 +6,7 @@
     }
 
     location ~* \.(?:ico|css|js|gif|jpe?g|png)$ {
-        expires 7d;
-        add_header Cache-Control "must-revalidate";
+        add_header Cache-Control "public, max-age=604800";
     }   
 
     # Add trailing slash to */wp-admin requests.