Address some review comments.
Split flocker.node.Port and flocker.node.gear.PortMap.

https://github.com/ClusterHQ/flocker/commit/fd686feadf363f4a1ac2cf044076c0670b6364b9#commitcomment-6998679
suggests that they are at a different level of abstraction, and so it would be
clearer if they are separate, even if they have the exact same structure.