Pass variables from deployment project services to CI runner ## What does this MR do? This commit introduces the concept of deployment variables - variables that are collected from deployment services and passed to CI runner during a deployment build. Deployment services specify the variables by overriding "predefined_variables" method. This commit also configures variables for KubernetesService ## Why was this MR needed? We need these values for https://gitlab.com/gitlab-org/gitlab-ce/issues/23580 ## Does this MR meet the acceptance criteria? - [x] [Changelog entry](https://docs.gitlab.com/ce/development/changelog.html) added - [x] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md) - ~~[ ] API support added~~ - Tests - [x] Added for this feature/bug - [ ] All builds are passing - [x] Conform by the [merge request performance guides](http://docs.gitlab.com/ce/development/merge_request_performance_guidelines.html) - [x] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides) - [x] Branch has no merge conflicts with `master` (if it does - rebase it please) - [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits) ## What are the relevant issue numbers? Refers to https://gitlab.com/gitlab-org/gitlab-ce/issues/23580 See merge request !8107
