Use cgroupsv2 for L3 cleanup

During OpenStack upgrades we need to ensure that no process is running
with the old code base. For that we used to kill L3 agent processes
except ones running haporoxy/keepalived.

However that handler was broken with migration of systems to cgroupsv2
as it was relying on a filepath for cgroups.
As all modern operating systems already migrated to cgroupsv2 or use
them in compatability mode, let's detect the actual path and use it
instead of the hardcoded one.

Change-Id: I717f3cf13a8001a1f2077a325c4ec451f45f9f74
(cherry picked from commit 6e23144f4847cfbe01115985e90afc65181800ce)

handlers/main.yml

@@ -60,7 +60,8 @@
 # orphaned processes are cleaned up correctly.
 - name: Run neutron-l3-agent process cleanup
   shell: |
-    for ns_pid in $(cat /sys/fs/cgroup/pids/neutron.slice/neutron-l3-agent.service/cgroup.procs); do
+    cgroup_path=$(findmnt -t cgroup2 -r -n -f -o target)
+    for ns_pid in $(cat ${cgroup_path}/neutron.slice/neutron-l3-agent.service/cgroup.procs); do
       echo $(readlink -f "/proc/$ns_pid/exe") | egrep -qv "keepalived|haproxy"
       if [ $? -eq 0 ] || [ "{{ neutron_l3_cleanup_on_shutdown | bool }}" = "True" ]; then
         if kill -9 "$ns_pid"; then