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Use a []string for CgroupName, which is a more accurate internal representation
**What this PR does / why we need it**:
This is purely a refactoring and should bring no essential change in behavior.
It does clarify the cgroup handling code quite a bit.
It is preparation for further changes we might want to do in the cgroup hierarchy. (But it's useful on its own, so even if we don't do any, it should still be considered.)
**Special notes for your reviewer**:
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.)
**NOTE**: I only tested this with dockershim, we should double-check that this works with the CRI endpoints too, both in cgroupfs and systemd modes.
/assign @derekwaynecarr
/assign @dashpole
/assign @Random-Liu
**Release note**:
```release-note
NONE
```
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