Add configuration of access_level for runners on registration

What does this MR do?

Allows users to use the gitlab-runner register --access-level="ref_protected" to be able to set the access-level at registration rather than via the UI/API. This speeds up the registration process and also helps to close a minor security issue between the registration of a runner and the time when it is set to protected via UI/API.

See https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/27490 for the Rails side of the code to accept the API request from gitlab-runner

Why was this MR needed?

See #3186 (closed)

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

Does this MR meet the acceptance criteria?

• Documentation created/updated
• Added tests for this feature/bug
• In case of conflicts with master - branch was rebased

What are the relevant issue numbers?

Closes #3186 (closed)