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>
Should we also change the code from line 18 in this file to ensure that it is always set?