Introduce default: for gitlab-ci.yml

What does this MR do?

EE version of https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/24627.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content