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)
Ack. With systemd, there will be another slice argument. Another option is to just take a config file like Create(). In practice, I found the smaller flagset much faster to work with.

I assumes that 'parent/child' like name would not be acceptable :)