Add a standalone test utility for cgroup package.
Although it has some overlap with nsinit utility, it is useful to test
cgroup functinality directly and try out new or internal features not
exposed through nsinit.

It still needs some work as I haven't added enough systemd support yet.
I also need to expose an enter method.

Docker-DCO-1.1-Signed-off-by: Rohit Jnagal <jnagal@google.com> (github: rjnagal)