Launch one tester per test.

Because sale trade condtion is already applied when it is set.

Suggested by Cédric de Saint Martin <cedric.dsm@tiolive.com>

xpath expression is modified, <tbody> is add to the filter
expression in order to do not follow any <tr> found inside <thead>.

This reverts commit 00475f4f.

Used in person_module, web_page_module, sale_order_module and portal_simulation

apply_load_balancing_scalability script is added to portal_skins/custom, in order
to perform load balancing. the script is based on zope port number to determine
on wich group each node have to be moved.