cgroups processors should have a flag to disable lookup of (root?) v2 cgroups

In cases where we report a `PID XXX contains a cgroups V2 path (0::/) but no V2 mountpoint was found.\nThis may be because metricbeat is running inside a container on a hybrid system.\nTo monitor cgroups V2 processess in this way, mount the unified (V2) hierarchy inside\nthe container as /sys/fs/cgroup/unified and start the system module with the hostfs setting.` error, we might want to actually provide a way for users to just disable v2 lookup, particularly if we're just stuck on the root `0::/`.

On some systems, getting the V2 hierarchy mounted is needlessly painful, and unnecessary if the container is only using V1, the V2 cgroups won't actually be doing anything.