Add support for init_params per device

plugins_params is currently a device property of the control device that is broadcasted to each/every devices that belong to the app.

Multiple options:

• One config file with all the init params as plugins_params (a la PSI)
• a vector plugin params with configuration for each devices (a la Smartpix)
• each device has it's own plugin_params, aka not broadcasted anymore (a la Sam :-))

@ponsard has a PR ready to implement option 2. @alejandro.homs what do you think?