How to start gnome wayland desktop from lightdm?
A bug report has been filed and is currently being worked on. There are a few workarounds but it isn't a perfect fix. I was able to login using Wayland with this fix but I was not able to use logout.
https://bugs.launchpad.net/ubuntu/+source/lightdm/+bug/1632772
One solution is to remove Unity 8 desktop:
sudo apt remove unity8-desktop-session
sudo apt autoremove