[2.3, trunk] MAAS power types should be cached by the regiond and not asked all the time

The regiond should cache the power types that a rack controller has instead of always asking it over and over again.

At the moment the regiond asks the rackd what its power types anytime it needs that information. That information is unlikely to change unless the rackd is reset.

The regiond should cache this information linked to each rackd connection, and once that connection is dropped to clear that information. This will ensure that data is always up to date, will require no database work, and can be backported to 2.3.