Automatic merge from submit-queue (batch tested with PRs 66190, 66871, 66617, 66293, 66891). If you want to cherry-pick this change to another branch, please follow the instructions <a href="https://github.com/kubernetes/community/blob/master/contributors/devel/cherry-picks.md">here</a>.
Do not set cgroup parent when --cgroups-per-qos is disabled
When --cgroups-per-qos=false (default is true), kubelet sets pod
container management to podContainerManagerNoop implementation and
GetPodContainerName() returns '/' as cgroup parent (default cgroup root).
(1) In case of 'systemd' cgroup driver, '/' is invalid parent as
docker daemon expects '.slice' suffix and throws this error:
'cgroup-parent for systemd cgroup should be a valid slice named as \"xxx.slice\"'
(5fc12449d8/daemon/daemon_unix.go (L618))
'/' corresponds to '-.slice' (root slice) in systemd but I don't think
we want to assign root slice instead of runtime specific default value.
In case of docker runtime, this will be 'system.slice'
(e2593239d9/daemon/oci_linux.go (L698))
(2) In case of 'cgroupfs' cgroup driver, '/' is valid parent but I don't
think we want to assign root instead of runtime specific default value.
In case of docker runtime, this will be '/docker'
(e2593239d9/daemon/oci_linux.go (L695))
Current fix will not set the cgroup parent when --cgroups-per-qos is disabled.
```release-note
Fix pod launch by kubelet when --cgroups-per-qos=false and --cgroup-driver="systemd"
```
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