Added systemd service-handling to 3.5

- Added a custom service_method to select service bundles for
  distributions which don't use the standard Sys-V init scripts,
  currently only systemd.
The `kept_successful_command classes` body is used to ensure that if the command executed returns 0 then the promise is recorded as kept instead of repaired. You should use it any where a commands promise is "kept" if the command returns 0.