Symfony2 configuration conditionally loading

My request change the configuration method for loading Symfony2 bundle.

If you configure ```saga``` repository to ```in_memory``` all ```mongodb``` service will not be loaded.

In same spirit, if you set the ```readmodel``` repository to ```in_memory```, the elasticsearch service will not be loaded.

So, if you want change for use another readmodel repository, you can set ```readmodel``` repository to ```in_memory``` dans overwrite alias ```broadway.read_model.repository_factory``` in your configuration file.

Open to suggestions and comments.