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>
Always join fs.Cpuset reguardless of value

Signed-off-by: Michael Crosby <crosbymichael@gmail.com>
Merge pull request #271 from crosbymichael/always-set-cpuset

libcontainer: setup cpuset cgroup by default