Merge branch 'expose-deployment-variables' into 'master'
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
