Forbid caching / for fuel-web

New header 'expires -1' forbids caching of / (index.html)
to avoid caching issues which may exist after master node
upgrade. This header is not needed for /static - for JS and
other assets Fuel UI uses its own cachebusting mechanism.

Change-Id: Ic9a199394a0167a4da977b5d135caf676e7d2b67
Closes-Bug: #1475310

deployment/puppet/nailgun/templates/nginx_nailgun.conf.erb

@@ -54,6 +54,7 @@ server {
     }
 
     location / {
+        expires -1;
         root <%= @staticdir %>;
     }