node: fix default kubelet/runtime cgroups when kube_reserved is false (#9834)

* node: fix default kubelet/runtime cgroups when kube_reserved is false (default)

Commit 1c4db613 introduced a notion of
kube_reserved. This introduced a breaking change defaulting to use
kube.slice for the container_manager and the kubelet as if kube_reserved
was always enabled whereas it is disabled by default.

This commit fixes this by bringing back system.slice whenever
kube_reserved is disabled.

Signed-off-by: Arthur Outhenin-Chalandre <arthur.outhenin-chalandre@proton.ch>

* inventory/sample: change false for kube_reserved as its the default

Changing the commented value in sample inventory to the actual default
value.

Signed-off-by: Arthur Outhenin-Chalandre <arthur.outhenin-chalandre@proton.ch>

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Signed-off-by: Arthur Outhenin-Chalandre <arthur.outhenin-chalandre@proton.ch>