/etc/default/lxd-bridge only exists in LXD 2.0.x, all newer releases, starting with LXD 2.1 have a in-LXD network management API which replaces it.
Those are the lxc network commands and you can find out about all the options here:
http://lxd.readthedocs.io/en/latest/networks/
To pass public IPs from the host to the container, you can either have them added to your host and NAT to your container’s IP, or (usually cleaner), route the IPs or subnets to your bridge, using the ipv4.routes option on the bridge and then adding it to the container with something like ip -4 addr add ADDRESS/32 dev eth0 (or the distribution network equivalent of that).