Set `runner_type` for cluster/application

What does this MR do?

This assigns proper runner_type for cluster created runner.

This is needed, as without otherwise we will have to re-run the DB migration for migrating the runner_type that we shipped with Group Runners. This was discovered as part of: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/18825.

Does this MR meet the acceptance criteria?

• Tests added for this feature/bug
• Review
  • Has been reviewed by Backend

What are the relevant issue numbers?

None yet.