zdtm: Mark mountpoints as exclusive

This test does nasty things with the root it runs in, in
particular is kills and creates /dev/null which may result
in /dev/null become 'deleted' for the tests running in
paralles with this. The lattest is critical for uns flavor
as ghost /dev/null will not be re-created (mknod fails).
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

test/zdtm/live/static/mountpoints.desc

-{'flavor': 'ns', 'flags': 'suid'}
+{'flavor': 'ns', 'flags': 'suid excl'}