Get a Cube's SSH connection info

Host (server public IPv4), host SSH port, login user, and the Cube's captured SSH host public keys. Use user rather than assuming a username — it is ubuntu or debian on Cubes created from images that ship a default user, and root on Cubes created before that change or imported from a rootfs without one. hostKeys is empty only until the reachability cron makes its first capture; clients fall back to trust-on-first-use until then, and pin strictly afterwards.