zdtm: add ability to execute non-namespacess tests concurrently

For that zdtm.sh is executed in pidns to avoid pid conflicts.

Cc: Christopher Covington <cov@codeaurora.org>
Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>