Merge branch 'seed-runner-token' into 'master'

Allow users to seed the initial runner registration token using an environment variable

## What does this MR do?
Allow users to seed the initial runner registration token using an environment variable

## Are there points in the code the reviewer needs to double check?

- Naming, do we want to make sure we are clear it's the 'registration' token. Like GITLAB_RUNNER_REGISTRATION_TOKEN vs what I have now, GITLAB_RUNNER_TOKEN
- Not sure we've tested a seed fixture before, I just made up a directory structure for the test

## Why was this MR needed?

At the moment I want to use this in our idea to production demo:  https://gitlab.com/gitlab-org/gitlab-ce/issues/22190

This is useful for when runner is bundled with gitlab, like in a kubernetes stack, and we want the runner to be able to register with gitlab as soon as they both come up.

## Does this MR meet the acceptance criteria?

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry 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
  - [x] All builds are passing
- [ ] Conform by the [merge request performance guides](http://docs.gitlab.com/ce/development/merge_request_performance_guidelines.html)
- [ ] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [ ] Branch has no merge conflicts with `master` (if you do - rebase it please)
- [ ] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

See merge request !6642

1 files changed, 12 insertions, 11 deletions

diff --git a/doc/administration/environment_variables.md b/doc/administration/environment_variables.md
index b4a953d1ccc..76029b30dd8 100644
--- a/doc/administration/environment_variables.md
+++ b/doc/administration/environment_variables.md
@@ -13,17 +13,18 @@ override certain values.
 
 Variable | Type | Description
 -------- | ---- | -----------
-`GITLAB_ROOT_PASSWORD`        | string  | Sets the password for the `root` user on installation
-`GITLAB_HOST`                 | string  | The full URL of the GitLab server (including `http://` or `https://`)
-`RAILS_ENV`                   | string  | The Rails environment; can be one of `production`, `development`, `staging` or `test`
-`DATABASE_URL`                | string  | The database URL; is of the form: `postgresql://localhost/blog_development`
-`GITLAB_EMAIL_FROM`           | string  | The e-mail address used in the "From" field in e-mails sent by GitLab
-`GITLAB_EMAIL_DISPLAY_NAME`   | string  | The name used in the "From" field in e-mails sent by GitLab
-`GITLAB_EMAIL_REPLY_TO`       | string  | The e-mail address used in the "Reply-To" field in e-mails sent by GitLab
-`GITLAB_EMAIL_REPLY_TO`       | string  | The e-mail address used in the "Reply-To" field in e-mails sent by GitLab
-`GITLAB_EMAIL_SUBJECT_SUFFIX` | string  | The e-mail subject suffix used in e-mails sent by GitLab
-`GITLAB_UNICORN_MEMORY_MIN`   | integer | The minimum memory threshold (in bytes) for the Unicorn worker killer
-`GITLAB_UNICORN_MEMORY_MAX`   | integer | The maximum memory threshold (in bytes) for the Unicorn worker killer
+`GITLAB_ROOT_PASSWORD`                     | string  | Sets the password for the `root` user on installation
+`GITLAB_HOST`                              | string  | The full URL of the GitLab server (including `http://` or `https://`)
+`RAILS_ENV`                                | string  | The Rails environment; can be one of `production`, `development`, `staging` or `test`
+`DATABASE_URL`                             | string  | The database URL; is of the form: `postgresql://localhost/blog_development`
+`GITLAB_EMAIL_FROM`                        | string  | The e-mail address used in the "From" field in e-mails sent by GitLab
+`GITLAB_EMAIL_DISPLAY_NAME`                | string  | The name used in the "From" field in e-mails sent by GitLab
+`GITLAB_EMAIL_REPLY_TO`                    | string  | The e-mail address used in the "Reply-To" field in e-mails sent by GitLab
+`GITLAB_EMAIL_REPLY_TO`                    | string  | The e-mail address used in the "Reply-To" field in e-mails sent by GitLab
+`GITLAB_EMAIL_SUBJECT_SUFFIX`              | string  | The e-mail subject suffix used in e-mails sent by GitLab
+`GITLAB_UNICORN_MEMORY_MIN`                | integer | The minimum memory threshold (in bytes) for the Unicorn worker killer
+`GITLAB_UNICORN_MEMORY_MAX`                | integer | The maximum memory threshold (in bytes) for the Unicorn worker killer
+`GITLAB_SHARED_RUNNERS_REGISTRATION_TOKEN` | string  | Sets the initial registration token used for GitLab Runners
 
 ## Complete database variables