Add handoff.ip to core cuttlefish schema

Items for discussion:
 * Included a validator prohibiting loopback per issue report — seems like a sane choice to me since handoff on the loopback interface is only useful for devrels, and in that case 0.0.0.0 is fine too.
 * I'm not clear on if/how validators are currently tested, so I've neglected to write a test for that as yet.