Revert the system_id generation

What does this MR do?

In !417 (merged) we introduced the automatic generation of the system_id to prevent system_id cluttering in the UI. The change made didn't take in account the use of replicas when deploying GitLab Runner with Helm Chart.

As consequence, when Values.replicas > 1, all the Pods registered use the same system_id.

A fix is not straightforward because of the nature of k8s deployments/replicasets. In order to unblock users, it has been decided to revert the changes while we think about a more robust solution.

Why was this MR needed?

To allow a system_id per replica

What's the best way to test this MR?

• Set Values.replicas to 2
• Install GitLab Runner
• SSH each GitLab Runner Pod and validate that the system_id are all differents

What are the relevant issue numbers?

close #556 (closed)