WIP: Option to disable service for systemd.

[130s]

Status of PR

WIP, as it is untested yet.

Problem

When provider is set to systemd, robot_upstart doesn't start the installed systemd unit and instead it only shows a comment to start manually, which I find is nice. However, (I guess by placing a unit file under /lib/systemd/system?) that unit is already enabled. So when the next time OS is rebooted, the systemd service in question starts. Sometimes this is not desired. User might want to get a control on whether the service is enabled or not.

Change in this PR

• Add disable option, with which set to true the systemd unit gets "installed" but disabled so the service won't start before it's explicitly enabled.

Open question

• Do we need to implement that for upstart?
  • Should upstart be still supported? This PR is targeted kinetic branch.