quantize discount/surcharge percentage values

It makes no sense to have discount/surcharge with 3 or more decimal points.
So, when setting sale.discount_percentage/sale.surcharge_percentage, quantize
the value generated so we will avoid generating that 3rd decimal point by
mistake.

Add some tests that would break without the change.

Fixes http://bugs.async.com.br/show_bug.cgi?id=5417
Change-Id: Ic78df12596979b313c591a15adadd65684228e89
Merge "quantize discount/surcharge percentage values"