Add predefined CI variables to GitLab

What does this MR do?

This adds predefined CI variables to GitLab for container registry, pipelines, project name, etc. It also makes sure that all currently documented variables are send from GitLab. This is added to follow up on this proposal: https://gitlab.com/gitlab-org/gitlab-ci-multi-runner/issues/185#note_11844286. To migrate almost all variables out of Runner to GitLab to simplify adding a new of variables without the need for changing the GitLab Runner.

Why was this MR needed?

Our CI variables miss a lot of crucial information that should be easily accessible. This tries to fill this gap.

What are the relevant issue numbers?

Fixes https://gitlab.com/gitlab-org/gitlab-ci-multi-runner/issues/185, https://gitlab.com/gitlab-org/gitlab-ce/issues/18164, https://gitlab.com/gitlab-org/gitlab-ce/issues/18075.

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together