Currently, Cinder only does cgroups v1. Let's use the cgroups
v2 command line, when it's available in /sys. This avoids
having to boot with compatibility kernel command line options:
systemd.unified_cgroup_hierarchy=false
systemd.legacy_systemd_cgroup_controller=false
and just work as expected, provided cgroup-tools >= 2.0.0 is
installed in the system.
Change-Id: Ifdfcd480b72727ec182d5a6954c706f365247edc
d41586a115