Fix _compute_qty() ceiling method

Consider only 3 digits after UoM rounding precising when apply ceiling
Hardcording a value (regardless of the value) sounds brittle. It is possible to find values and roundings that would offset the result too much or wouldn't offset it enough; keep in mind that roundings can be quite small or quite large, and not necessarily powers of 10 either.

This is why we have to compute a dynamic `epsilon` value in [float_round()](https://github.com/odoo/odoo/blob/759ccd984542c/openerp/tools/float_utils.py#L67) and why we have [extensive tests](https://github.com/odoo/odoo/blob/759ccd984542ce4077dfcc9fa14198b60719dded/openerp/addons/base/test/base_test.yml#L201) for it.

Perhaps a solution is to implement this [TODO](https://github.com/odoo/odoo/blob/1e048736df81d58e/addons/product/_common.py#L31) by adding a `rounding_method` option to `float_round()`, and use `rounding_method=UP` to implement `ceiling()`. This would require extending the `float_round()` tests to validate this extra use case.