LXD and any sort of cluster seem incompatible at the moment. LXD, for example, stores all the LXC config info in a local sqlite database. When you launch a container, it turns the sqlite stored info into an LXC config file, and spawns LXC.
You can control remote instances of the LXD daemon, but there's no syncing back and forth from Host1-LXD1 to Host1-LXD2...they operate separately. You can migrate a container from one host to the other, but it's pushing everything (config/image/etc) when you ask it to migrate.
So, it's not clustered itself, and it keeps the important info somewhere that's hard to cluster.
It would be interesting if they would replace the local sqlite store with something similar to etcd. That still leaves the issue of the disk images though.
Proxmox does cluster LXC, so there's that option.