Support installing strong-pm as a systemd service

Use systemd support in strong-service-install@1.1.0 to install
strong-pm as a systemd Service with the same level of support as
Upstart.

 * Depends on strongloop/strong-service-install#5
 * Closes #25
 * Closes strongloop-internal/scrum-nodeops#19
", see `%s --help`."