Bug#943981: Proposal: Switch to cgroupv2 by default

[Ryutaroh Matsumoto]

I verified that
* commenting out lxc.init.cmd and
* putting lxc.mount.auto = cgroup:rw:force  AT THE LAST LINE of
   /var/lib/lxc/autopkgtest-unstable-amd64/confiig

allowed successfull autopkgtest of systemd on a host both with
unified and hybrid hierarchy, WITHOUT changing
/var/lib/lxc/autopkgtest-unstable-amd64/confiig.

A problem is that putting lxc.mount.auto = cgroup:rw:force in /etc/lxc/default.conf
will not work, because the content of /etc/lxc/default.conf is overwritten by
lxc.include = /usr/share/lxc/config/debian.conf
loaded later in autopkgtest-unstable-amd64/confiig.

On the other hand
> lxc.mount.auto = cgroup:rw:force proc:mixed sys:mixed
> in /usr/share/lxc/config/common.conf.
should work.

Ryutaroh