zdtm/cgroup02: remove subgroups in the cleanup hook

systemd executes tests in subgroups:
 9679 ?        Ssl    0:41 /etc/alternatives/java -Dcom.sun.akuma.Daemon=daemonized -Djava.awt.headless=true -DJENKINS_HOME=/var/lib/jenkins -jar /usr/lib/jenkins/jenkins.war -
[root@jenkins ~]# cat /proc/9679/cgroup
10:net_cls:/
9:hugetlb:/
8:cpuset:/
7:blkio:/system.slice
6:freezer:/
5:cpu,cpuacct:/system.slice
4:devices:/system.slice/jenkins.service
3:perf_event:/
2:memory:/system.slice
1:name=systemd:/system.slice/jenkins.service

Reported-by: Mr Jenkins
Signed-off-by: Andrew Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>