fix path of pcmk-stack-restart exec object, service can be below /usr, too

The pcmk-stack-restart exec object only has /bin and /sbin specified as path, Ubuntu hovever has service(8) located in /usr/sbin. This patch therefore adds /usr/bin and /usr/sbin to the path, making restarting pacemaker work on Ubuntu (and Debian, as long as the service (8) has been installed).