libcontainer: setup cpuset cgroup by default

Currently if we don't use --cpuset, the cpuset cgroup is not
created, it's bad if we want to modify cpuset config subsequently,
change the behavior to make it right.

Signed-off-by: Qiang Huang <h.huangqiang@huawei.com>
@crosbymichael Yes, I think so, it's the fs way we need to fix too.
I use systemd so just tested systemd way.

I'm not available right now, can you merge this first? and I'll fix the other
in a subsequent patch as soon as I can test it.
If you need this to be done in the same patch, you can hold it, I'll resend
it ASAP. :)

Thanks.
Qiang