Fix update-engine failed to write cpu.shares

We now using systemd and will create cgroup controller for update-engine in
systemd way to prevent below error.

[1208/053539:INFO:utils.cc(633)] Setting cgroup cpu shares to  2
[1208/053539:ERROR:utils.cc(118)] 0 == writer.Open(path, O_WRONLY | O_CREAT | O_TRUNC, 0600) failed: No such file or directory
[1208/053539:ERROR:utils.cc(638)] Failed to change cgroup cpu shares to 2 using /sys/fs/cgroup/cpu/update-engine/cpu.shares

Signed-off-by: Chengwei Yang <yangchengwei@qiyi.com>