MINOR: systemd: Use variable for config and pidfile paths

Users can set the location of haproxy.cfg and pidfile files by providing
a systemd overwrite file /etc/systemd/system/haproxy.service.d/overwrite.conf
with the following content:

    [Service]
    Environment=CONFIG=/etc/foobar/haproxy.cfg
(cherry picked from commit cc23a79f34cc5a34572526e8f13b97e28dd09aca)
(cherry picked from commit fd532376fbeaf4189a52bf4d18e91c912c6fd7ce)
1 file changed