Make Tahoe-LAFS storage server IP and port configurable in the private-storage.nix module

Created by: exarkun

Storage servers need to know their own address so they can advertise it.

Actually, they only really need to know it if they're going to be used with an introducer or you want the private/storage.furl they write to be correct. These might not be technically required but they're both nice to have and I already did the work for this in a branch for another issue :/

So I guess it may as well be checked in.