roles/nginx: Add index to vhost config

Without this, all requests to directory URIs throw 403 errors due
to directory listings not being allowed.

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

roles/nginx/templates/vhost.conf.j2

@@ -8,6 +8,8 @@ server {
 
     server_name {{ domain_name }} {{ domain_aliases }};
 
+    index index.php index.html;
+
     access_log  /var/log/nginx/{{ domain_name }}-access.log;
     error_log   /var/log/nginx/{{ domain_name }}-error.log;