I did not find any hint about it...
Is it possible to run Ubuntu 15.10 in a lxc container on an Ubuntu 14.04LTS host?
Yes it is possible to run an Ubuntu 15.10 container in a Ubuntu 14 host. Use the steps in this site. The main change is when creating your container you need to specify the exact image to download by its code name (ubuntu 15.01 is "vivid vervet").
sudo lxc-create -t download -n u1 -- -d ubuntu -r vivid -a amd64
That and you are good to go. Remember a container is (just) a namespace as long as it is GNU/Linux.