Use configuration template file feature for configuring Runner on registration

⚠ Prepared on top of !165 (merged) ⚠

With gitlab-org/gitlab-runner!1263 (merged) we've added a way to pass configuration at registration time in a more clean way than by using CLI options or - what's even less nice - environment variables.

This MR refactors the way how currently supported options are passed to the newly registered Runner, to use the new feature.

Closes #106 (closed)