Comment 1 for bug 1533833

You're using a newer kernel which provides the 'pids' cgroup. Systemd doesn't
know about that one and so doesn't create a cgroup for you that you own. Lxc
in turn (in wily) doesn't yet know how to handle that.

You can work around this several ways. The simplest is to do

sudo cgm create pids user
sudo cgm chown pids user $(id -u) $(id -g)
cgm movepid pids user $$

before you start the container.