Use value of `yaml_variables` and `when` from config_processor if undefined

What does this MR do?

Uses yaml_variables and when from config_processor if values for these are undefined.

Why was this MR needed?

Old builds doesn't have this columns initialised. Thus makes the retries on these builds to not work properly. This is regression introduced by https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/5296.

Does this MR meet the acceptance criteria?

• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)