7.0 fix 2854 product reserve round floor

This fixes https://github.com/odoo/odoo/issues/2854. 
PR on Odoo https://github.com/odoo/odoo/pull/3457

By default, the rounding method in product_uom round amount with ceiling. Using a ceiling rounding method in product reserve can let the user reserve more than what is available in a location. Using floor rounding in that situation is safe and will prevent negative stocks.