tests: prevent zdtm_mount_cgroups from failing w/ EBUSY

Sometimes I get:

sudo ./zdtm.py run -t zdtm/live/static/cgroup03
umount: /home/tycho/packages/criu/test/zdtm.qPwsoO: target is busy
        (In some cases useful info about processes that
         use the device is found by lsof(8) or fuser(1).)
Traceback (most recent call last):
  File "./zdtm.py", line 1401, in <module>
    tst.available()
  File "./zdtm.py", line 435, in available
    subprocess.check_call(["flock", "zdtm_mount_cgroups", "./zdtm_mount_cgroups"])
  File "/usr/lib/python2.7/subprocess.py", line 540, in check_call
    raise CalledProcessError(retcode, cmd)
subprocess.CalledProcessError: Command '['flock', 'zdtm_mount_cgroups', './zdtm_mount_cgroups']' returned non-zero exit status 1

let's use a lazy unmount so we avoid this.

Signed-off-by: Tycho Andersen <tycho.andersen@canonical.com>
Acked-by: Andrew Vagin <avagin@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>