Chassis power types should be dynamically listed

[Treydone]

All the chassis power types are listed in ui/src/app/machines/views/AddChassis/chassisPowerTypes.js. If we add a new chassis power types, we have to change the backend and the ui. A better way is to use the settings not on scope SETTING_SCOPE.NODE registered in each driver and available in /MAAS/api/2.0/machines/?op=power_parameters

[squidsoup]

Thanks for submitting an issue @Treydone, that's certainly something we should investigate!

[Caleb-Ellis]

Each power type returned from general.power_types now has a boolean field chassis which we can use to get a list of chassis power types.