Allow to use other connection and entity manager than "default"

[phansys]

Subject

Allow to use other connection and entity manager than "default".

I am targeting this branch, because these changes respect BC.

Closes #140.

Changelog

### Added
- `connection` configuration node in order to use a different connection than "default";
- `entity_manager` configuration node in order to use a different entity manager than "default".

[VincentLanglet]

Thanks @phansys