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](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [ ] Branch has no merge conflicts with `master` (if you do - rebase it please)


See merge request !5342