MINOR: systemd: support /etc/sysconfig/ for redhat based distrib

The patch "MINOR: systemd: Make use of master socket in systemd unit"
introduces an environment file in /etc/default.

Unfortunatly this is not supported on redhat-based system, so we add
/etc/sysconfig/haproxy for that.
diff --git a/contrib/systemd/haproxy.service.in b/contrib/systemd/haproxy.service.in
index 44c369c..9b7c3d1 100644
--- a/contrib/systemd/haproxy.service.in
+++ b/contrib/systemd/haproxy.service.in
@@ -4,6 +4,7 @@
 
 [Service]
 EnvironmentFile=-/etc/default/haproxy
+EnvironmentFile=-/etc/sysconfig/haproxy
 Environment="CONFIG=/etc/haproxy/haproxy.cfg" "PIDFILE=/run/haproxy.pid" "EXTRAOPTS=-S /run/haproxy-master.sock"
 ExecStartPre=@SBINDIR@/haproxy -f $CONFIG -c -q $EXTRAOPTS
 ExecStart=@SBINDIR@/haproxy -Ws -f $CONFIG -p $PIDFILE $EXTRAOPTS