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 enter and notification methods; will get to that if this looks okay.

Docker-DCO-1.1-Signed-off-by: Rohit Jnagal <jnagal@google.com> (github: rjnagal)
nit: can we say this is a cgroups.Cgroup object? Just wondering how the user knows what config to use.