ftests/071: Introduce delay before killing cgroup

ftests/071: Introduce delay before killing cgroup

Fix the following issue:
071-sudo-set_default_systemd_cgroup.py - cgroup_delete_cgroup failed: 50016

Some older version of Kernel/Python combination, requires few jiffies to
sync the cgroup.procs list of process. Fix it by introducing 0.5 second
delay, before removing the cgroup.

Signed-off-by: Kamalesh Babulal <kamalesh.babulal@oracle.com>
Signed-off-by: Tom Hromatka <tom.hromatka@oracle.com>