Closes #2028 

- Refactor: move fixtures to corresponding directories
- Delete unused fixtures

Resolves: #3328 

Manually finds and massages node updates in Docker, Docker Compose and Circle CI so that they should take on the same “renovate/node-8.x” style branch naming. The goal is to unify all node updates into a single branch.

Adds support for renovating Docker Compose files (e.g. `docker-compose.yml`). Functionality is essentially the same as the existing `Dockerfile` capabilities, so config for `docker` is shared with `docker-compose` but may also be overridden.

Merging as disabled by default - will wait for some opt-in testing before turning it on by default.

Closes #832