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

1 files changed, 11 insertions, 0 deletions

diff --git a/doc/project_services/kubernetes.md b/doc/project_services/kubernetes.md
index cb577b608b4..fda364b864e 100644
--- a/doc/project_services/kubernetes.md
+++ b/doc/project_services/kubernetes.md
@@ -36,3 +36,14 @@ to create one. You can also view or create service tokens in the
 Fill in the service token and namespace according to the values you just got.
 If the API is using a self-signed TLS certificate, you'll also need to include
 the `ca.crt` contents as the `Custom CA bundle`.
+
+## Deployment variables
+
+The Kubernetes service exposes following
+[deployment variables](../ci/variables/README.md#deployment-variables) in the
+GitLab CI build environment:
+
+- `KUBE_URL` - equal to the API URL
+- `KUBE_TOKEN`
+- `KUBE_NAMESPACE`
+- `KUBE_CA_PEM` - only if a custom CA bundle was specified