BUG/MINOR: systemd: must check the configuration using -Ws

BUG/MINOR: systemd: must check the configuration using -Ws

When doing a reload with a configuration which requires the
master-worker mode, the configuration check will fail because the check
is not done with -W/-Ws.

Example:
	wla@kikyo:~/haproxy$ ./haproxy -Ws -c -f haproxy.cfg
	Configuration file is valid
	wla@kikyo:~/haproxy$ ./haproxy -c -f haproxy.cfg
	[NOTICE]   (13153) : haproxy version is 2.5-dev2-4567b3-16
	[NOTICE]   (13153) : path to executable is ./haproxy
	[ALERT]    (13153) : config : Can't use a 'program' section without master worker mode.
	[ALERT]    (13153) : config : Fatal errors found in configuration.

This patch fixes the issue by adding -Ws on the check command line.

Must be backported in all stable branches. (The file was previously in
contrib/systemd/haproxy.service.in).