Refine support for CI/CD config entries that are arrays

The following discussion from gitlab-ce!8578 should be addressed:

• @grzesiek started a discussion:

  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.