No need of -d flag to specifiy directories

Implements both Plugin child classes handling extensions and session handling.
Modified the settings plugin preload piece of code. It is now adapted to the new plugins organisation.
Each Plugin child classes provides a confspec indicating where the plugin list is stored in configuration.

- add a -L options to list availables actions

- Allow to extend easily lodel_admin.py script.
- Writtent first lodel_admin.py action : discover-plugins