MINOR: systemd: Check configuration before start

Adds a configuration check before starting the haproxy service.
(cherry picked from commit 91a9f1a2e1421a376a4b63f9342addd59cb2b244)
diff --git a/contrib/systemd/haproxy.service.in b/contrib/systemd/haproxy.service.in
index 0bc5420..85937e4 100644
--- a/contrib/systemd/haproxy.service.in
+++ b/contrib/systemd/haproxy.service.in
@@ -3,6 +3,7 @@
 After=network.target
 
 [Service]
+ExecStartPre=@SBINDIR@/haproxy -f /etc/haproxy/haproxy.cfg -c -q
 ExecStart=@SBINDIR@/haproxy-systemd-wrapper -f /etc/haproxy/haproxy.cfg -p /run/haproxy.pid
 ExecReload=/bin/kill -USR2 $MAINPID
 KillMode=mixed