[EE Port] Renamed terminal_specification to channel_specification

What does this MR do?

This is an EE port of https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/26939

Does this MR meet the acceptance criteria?

Yes

Conformity

• Code review guidelines
• Merge request performance guidelines
• Style guides
• Separation of EE specific content

Performance and testing

• Tests added for this feature/bug