With the introduction of cgroup v2 there are new names used with
cgroups based on which version is used:

    - legacy: cgroup v1
    - unified: cgroup v2
    - hybrid: cgroup v1 and cgroup v2

Let's use 'legacy' instead of 'cgroupv1' or 'controllers' in our code.
Reviewed-by: NFabiano Fidêncio <fidencio@redhat.com>
Reviewed-by: NJán Tomko <jtomko@redhat.com>
Signed-off-by: NPavel Hrdina <phrdina@redhat.com>