Sandboxes for ecommerce don't pass along the scheme properly

The nginx config is only configured for prod (when it is behind an ELB
and has http_x_forwarded headers).  We want to pass along the NGINX
headers too (mainly scheme) whcih is what the LMS/CMS does on sandboxes.

playbooks/roles/nginx/templates/edx/app/nginx/sites-available/ecommerce.j2

@@ -66,9 +66,15 @@ server {
   {% include "robots.j2" %}
 
 location @proxy_to_app {
+    {% if NGINX_SET_X_FORWARDED_HEADERS %}
+    proxy_set_header X-Forwarded-Proto $scheme;
+    proxy_set_header X-Forwarded-Port $server_port;
+    proxy_set_header X-Forwarded-For $remote_addr;
+    {% else %}
     proxy_set_header X-Forwarded-Proto $http_x_forwarded_proto;
     proxy_set_header X-Forwarded-Port $http_x_forwarded_port;
     proxy_set_header X-Forwarded-For $http_x_forwarded_for;
+    {% endif %}
     proxy_set_header Host $http_host;
 
     proxy_redirect off;