Add a feature to manage the gitlab-runner service files

[EdgeJ]

Currently, the gitlab-ci-runner module manages the config.toml file for setting up the runners themselves. However, it would also be nice to manage the systemd/sysv service file as well to do things like set a different user to run the gitlab-runner service, set a different working directory, etc.

[bobweston]

I came here specifically to see if that use case was baked in to the module. (Change the gitlab-runner's user from gitlab-runner to another user.). Our devs need the runner to execute as non-root or gitlab-runner user.

I found a way (on stackoverflow) to do it manually. But we've tried to avoid making manual changes on our VMs.

https://stackoverflow.com/questions/37187899/change-gitlab-ci-runner-user