The rest of this diff is pretty straightforward, but this part is letting the caller select a PHID for a newly created interface. (Creating objects with preassigned PHIDs is uncommon, but not inherently problematic.)

This makes synchronizing service definitions from the administrative server to instance servers a lot easier. In turn, synchronizing means that we can just build the clustering code in a general-purpose way without giving it any special knowledge of Phacility or the management infrastructure.