cgroup: Fix None value check and modify the way to create a path with os.path.join()

Signed-off-by: Li Yang <liyang.fnst@cn.fujitsu.com>
cgroup: Add/Modify some functions in utils_cgroup.py

Signed-off-by: Li Yang <liyang.fnst@cn.fujitsu.com>
Merge pull request #689 from liyangfnst/cgroup_test

Cgroup test:None value check, os.path.join() form a path, add/modify  functions