MINOR: systemd: Use the variables from /etc/default/haproxy

MINOR: systemd: Use the variables from /etc/default/haproxy

This will allow seamless upgrades from the sysvinit system while respecting
any changes the users may have made. It will also make local configuration
easier than overriding the systemd unit file.

Note by Tim:

This GPL-2 licensed patch was taken from the Debian project at [1].

It was slightly modified to cleanly apply, because HAProxy's default unit
file does not include rsyslog.service as an 'After' dependency. Also the
subject line was modified to include the proper subsystem and severity.

This patch may be backported to 1.9.

[1] https://salsa.debian.org/haproxy-team/haproxy/blob/master/debian/patches/haproxy.service-use-environment-variables.patch

Co-authored-by: Tim Duesterhus <tim@bastelstu.be>