Without this a systemd container won't start:
> $ lxc-start -n vagrant-base-stretch-amd64
> Failed to mount cgroup at /sys/fs/cgroup/systemd: Permission denied
> [!!!!!!] Failed to mount API filesystems, freezing.
> Freezing execution.
See upstream commit: lxc-1.1.0.rc3-12-gf24a52d [Use consistent /proc, /sys and /sys/fs/cgroup (v2)]
The lxc-debian template aready does this, but apparently
the download template does not. We need to adapt the config
for installation as well as provide a version-specific config
for packaging the box.