core: don't use the unified hierarchy for the systemd cgroup yet (#4628)

Too many things don't get along with the unified hierarchy yet:

 * https://github.com/opencontainers/runc/issues/1175
 * https://github.com/docker/docker/issues/28109
 * https://github.com/lxc/lxc/issues/1280

So revert the default to the legacy hierarchy for now. Developers of the above
software can opt into the unified hierarchy with
"systemd.legacy_systemd_cgroup_controller=0".
(cherry picked from commit 843d5baf)