optionally provision additional runners

NOTE: Must provide a RUNNER token, not the REGISTRATION token shown on the gitlab runner webpage. To get a runner token from a registration token:

$ curl -d "token=$REGISTRATION_TOKEN" https://gitlab.com/api/v4/runners

Use by creating or editing /etc/ansible/facts.d/additional_gitlab_runners.fact:

{
  "runners": [
    {
      "domain": "gitlab.com", // optional, defaults to the domain configured in gitlab_runner.fact
      "token": "RUNNER token",
      "name": "signald" // this is to tell the different runners on this machine apart. It should be the name of the gitlab project.
    }
  ]
}