Validate numericness of ports in builder files.

You can't really goof this up using bin/swift-ring-builder, but if you
have code that uses swift.common.ring.RingBuilder directly, you can
stuff e.g. "6002" in where you mean 6002, resulting in some fairly
baffling failures. (Yes, I have done this.)

Change-Id: I87b7b7066b9ea2ce6f82255605da99cf0d283689