Allows to switch a service from a machine to another without having to reconfigure backups. Very useful for migrations for example.

Basically :
- When you push a Dockerfile, all the pipeline will be triggered, except the push-prod stage.
- When you push a docker-compose, only the security checks will be done, based on the latest built image. It is in that case useless to rebuild the image!
- When you push a clair-whitelist, only the Clair stage will be done. Same reason as above.
- When you push something else (e.g. a config file), you may trigger any step you want by hand, for flexibility. No more dumb "Trigger CI" commits!
- When you push something else (not in a pica-* folder), nothing happens and you cannot trigger anything. This is expected.

The pipeline will automatically build the image whenever some files are changed.

But for some tasks, you may want to enable certain stages manually.
This is what this commit does.