Refine support for CI/CD config entries that are arrays
The following discussion from gitlab-ce!8578 should be addressed: - [ ] @grzesiek started a [discussion](https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/8578#note_32292293): > We should have a first-class support for entries that can be arrays. Currently `Configurable` entries can be only Hashes, but the same pattern for an array makes sense. We should either extend `Configurable` or ship this as a separate mix-in / something else.
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