adding support for kubernetes service account and override on gitlab-ci.yaml

What does this MR do?

Adds support to specify and override the kubernetes system account of the kubernetes executor pod

Why was this MR needed?

In a cluster with RBAC installed, if the namespace is overwritten via KUBERNETES_NAMESPACE_OVERWRITE the job will fail because the SA of the runner won't be available on the target namespace.

Are there points in the code the reviewer needs to double check?

No

Does this MR meet the acceptance criteria?

  • Documentation created/updated
  • Tests
    • Added for this feature/bug
    • All builds are passing
  • Branch has no merge conflicts with master (if you do - rebase it please)

What are the relevant issue numbers?