hotfix docker rmi via id

Fix and issue in conflict detection in `docker rmi`
You can't rmi any tagged image by it's id

Now, it's check all the repo so see if the id belongs to one or multiple repositories.
`b750fe79269d` belongs to `base:latest`, `base:ubuntu-12.10`, `base:ubuntu-quantal`, `base:ubuntu-quantl`, so it belongs only to `base` 

So `docker rmi b750fe79269d` behave like `docker rmi base`

@creack @shykes please consider merging this asap