With the latest update of Varnish, the configuration file has moved to a new place. This is how you do it Debian 8 with varnish 4.1.4-1~jessie:
Create folder and insert in /etc/systemd/system/varnish.service.d/customexec.conf:
sudo -i
mkdir /etc/systemd/system/varnish.service.d
vi /etc/systemd/system/varnish.service.d/customexec.conf