[Warehouse] Prevent negative stock.

Task: https://accounts.openerp.com/web#id=10130&view_type=form&model=project.task&menu_id=3942&action=327
Pad: https://pad.odoo.com/p/r.KCBJWsVnxINg746v
yes you are right.
but how can we skip this module not to install itself on runbot.? we have to add skip module list on runbot branch like `mods = mods - set(['auth_ldap', 'document_ftp', 'hw_escpos', 'hw_proxy', 'hw_scanner','stock_negative'])`