A global var systemd_service_restart_changed is added which toggles
restart on unit file change for all systemd_services.
A systemd_services var 'restart_changed' is added to toggle restarts
on a per-service basis.
systemd daemon is not reloaded in a loop anymore, it is only
reloaded once, in a handler, when unit files change.
Consumer roles of systemd_service can listen to the handler
'systemd service changed' to implement additional orchestration when
service units are modified.
Change-Id: I803bca638c30696004f934ad1e28920786948f38
72efb59ad5