Support for deploying a shared runner for defined list of groups/projects

The upstream gitlab-runner is very versatile and can be shared between multiples instances or multiples groups/projects.

The current chart does not let us declare multiple registrations. The issue #222 seems to go in the direction of offering all the power of the upstream runner.

My main motivation is the resource sharing: having a single gitlab-runner registered on multiple groups/projects will ensure the concurrent setting is not exceeded.

Possible solutions:

• add support for multiple registrations elements (lot of impact to manage mutiple registration secrets and iteration)
• add support for multiple already-registered token (less impact?)
• add support for a predefined config.toml