[WIP] PHP platform frontend & interpretor configurable in `app.yml`

**Note:** this is a work-in-progress pull-request but reviews, comments are very welcome !

The idea here is to have a PHP platform that is a lot customisable though the `app.yml` file. There's a difference between frontend and interpretor:
- Frontend can be nginx or apache
- Interpretor can be php (though fpm) or hhvm

Steps:
- [x] Rewrite the platform to chose between frontend & interpretor in `app.yml` file
- [x] Create an `apache-mod-php` frontend as default to keep backward compatibility
- [ ] Create an `nginx` frontend
- [ ] Create an `fpm54` interpretor

I wrote a `README.md` file in which there're explanations about how to use it from a user point of view.