BrooklynNode cluster + upgrade effector

Also effectors to:
  * select master in the cluster
  * set HA priority
  * set HA state
Is this in `init` because you want it to be persisted? Two things:
 * the callable for the poll config needs to be serializable by xstream, so declare a static class that takes the entity in its constructor.
 * call `scanMaster = addFeed(FunctionFeed.builder()...)`. I believe that `addFeed` call is still required.
   (if it's not, then I suspect we'll be breaking a lot of our existing entities that have not have their feeds fixed!)