[HOTFIX] Added template key as template var

__Tasks:__

- [x] test coverage
- [ ] gather feedback for my changes

__Informations:__

| Q             | A
| ------------- | ---
| Tests pass?   | yes
| Fixed tickets | none
| BC Breaks     | no
| Doc           | https://github.com/sulu-cmf/docs/blob/master/developer-documentation/300-webspaces/templates.md