Tricky one. Not sure either. If you just have the spec here (i.e. delete 
the factory) it'll probably be fine and better/simpler code. But for 
DynamicCluster it's used in a lot of places and I'm not confident enough 
that all of them (including external repos using brooklyn) could easily 
use spec instead. Also there's legacy code that passes a factory and I 
didn't want to break that.