The nginx configuration was set to pass X-Forwarded-For and related headers
unodified to backend services.
This works fine when nginx is deployed behind another load balancer
(for example ELB), which already properly sets the X-Forwarded-* for headers.

But in simpler deployment scenarios, where nginx is directly facing the end user,
nginx should discard any existing X-Forwarded-For headr and set it to remote IP of the request,
the X-Forwarded-Port header to the nginx server port, and the X-Forwarded-Proto
to the scheme of the current request.

The 'NGINX_SET_X_FORWARDED_HEADERS' is set to False by default, but set to True
in the edx_sandbox playbook, where an additional load balancer in front of nginx
is not usually present.

Run some tasks as jenkins user so that the permissions are correct. Also some refactoring of the jenkins tasks.