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

This pullreq is about client/shared/utils_cgroup.py
Fix some None value check, modify the way to create a path with os.path.join(), add/modify some functions about cgroup in utils_cgroup.py
The relational pullreq is:
https://github.com/autotest/autotest-client-tests/pull/14