Add a cleanup method to cgroup fs. This will help in building a
standalone cgroup test util. systemd does auto cleanup on empty, so
doesn't need an extra interface.

Docker-DCO-1.1-Signed-off-by: Rohit Jnagal <jnagal@google.com> (github: rjnagal)
Merge pull request #74 from rjnagal/cgroup_testing

Add a cleanup method to cgroup fs. This will help in building a