The slice of strings more precisely captures the hierarchic nature of
the cgroup paths we use to represent pods and their groupings.

It also ensures we're reducing the chances of passing an incorrect path
format to a cgroup driver that requires a different path naming, since
now explicit conversions are always needed.

The new constructor NewCgroupName starts from an existing CgroupName,
which enforces a hierarchy where a root is always needed. It also
performs checking on the component names to ensure invalid characters
("/" and "_") are not in use.

A RootCgroupName for the top of the cgroup hierarchy tree is introduced.

This refactor results in a net reduction of around 30 lines of code,
mainly with the demise of ConvertCgroupNameToSystemd which had fairly
complicated logic in it and was doing just too many things.

There's a small TODO in a helper updateSystemdCgroupInfo that was
introduced to make this commit possible. That logic really belongs in
libcontainer, I'm planning to send a PR there to include it there.
(The API already takes a field with that information, only that field is
only processed in cgroupfs and not systemd driver, we should fix that.)

Tested by running the e2e-node tests on both Ubuntu 16.04 (with cgroupfs
driver) and CentOS 7 (with systemd driver.)