gitlab-runner: Tune check_interval and request_concurrency

Move values for concurrent and replicas into variables and create new variables for check_interval and request_concurrency. The latter two are set to values that should allow cloud runners to pick up jobs more quickly.

Set check_interval to 3 which incidentally is the default value of the field in gitlab-runner configuration but not in the gitlab-runner helm chart which has it defaulted to 30.

Ref https://gitlab.com/gitlab-org/charts/gitlab-runner/-/issues/148