Add online terminal support for Kubernetes

## What does this MR do?

Gives terminal access to kubernetes-deployed environments  via the deployment service

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

## Why was this MR needed?

Part of idea to production

## Screenshots (if relevant)

### `/root/reviewing/environments`

![Screen_Shot_2016-12-15_at_19.10.40](/uploads/bd2c54c07b6c85dec3328a20cd185b64/Screen_Shot_2016-12-15_at_19.10.40.png)

### `/root/reviewing/environments/10013`

![Screen_Shot_2016-12-19_at_12.52.39](/uploads/db4e4e06cda88437e8727433d65898b9/Screen_Shot_2016-12-19_at_12.52.39.png)

### `/root/reviewing/enviroments/10013/terminal`

![Screen_Shot_2016-12-15_at_02.35.52](/uploads/1bb77b7e2de2c657ae3bda62dc4f0970/Screen_Shot_2016-12-15_at_02.35.52.png)

## 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)
- Tests
  - [X] Added for this feature/bug
  - [x] 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?

* Closes #22864 #22958
* Alternative to, and somewhat based on,  !6770 
* Depends on https://gitlab.com/gitlab-org/gitlab-workhorse/merge_requests/83

See merge request !7690