overlay.py: Adds plug-in module controller

This commit implements the module controller system for the overlay
plug-in modules.
api.py: Makes supported_types() check for modules

Prior to executing require_supported() and notifying a user that the
overlay type isn't supported due to not finding the proper command
bin, a check has been made to see if the overlay module has been
brought in by the user. This will prevent unnecessary complaining from
layman.