DEVOPS-2074: nginx looks for ELB headers to forward

In the case of sandboxes, nginx listens on 443/80 and needs to pass
along the scheme of the request so that django knows if the client is
connecting over HTTP or HTTPS, rather than forwarding non-existant ELB
headers.

util/jenkins/ansible-provision.sh

@@ -138,6 +138,7 @@ EDXAPP_STATIC_URL_BASE: $static_url_base
 EDXAPP_LMS_NGINX_PORT: 80
 EDXAPP_LMS_PREVIEW_NGINX_PORT: 80
 EDXAPP_CMS_NGINX_PORT: 80
+NGINX_SET_X_FORWARDED_HEADERS: True
 EDX_ANSIBLE_DUMP_VARS: true
 migrate_db: "yes"
 openid_workaround: True