zdtm: move all logic about namespaces from test_init in libzdtm

A test will be executed in new set of namespaces if the environment
variable ZDTM_NEWNS is set. A pid of the root task will be written in
$ZDTM_PIDFILE and a root fyle system will be changed on $ZDTM_ROOT.

Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>