erp5_configurator_standard_invoicing_template: use ROUND_HALF_UP rounding option.

with ROUND_DOWN, we have a different behaviour between py2/py3, that caused failures in
erp5_simplified_invoicing:testTradeModelLine.

Here is what happened in _round(1.9999999999999998) in
bt5/erp5_simulation/DocumentTemplateItem/portal_components/document.erp5.FloatEquivalenceTester.py

* py2
decimal.Decimal(str(1.9999999999999998)).quantize(decimal.Decimal('0.000001'), 'ROUND_DOWN')
=> Decimal('2.000000')
(because str(1.9999999999999998) is '2.0')

* py3
decimal.Decimal(str(1.9999999999999998)).quantize(decimal.Decimal('0.000001'), 'ROUND_DOWN')
=> Decimal('1.999999')
(because str(1.9999999999999998) is '1.9999999999999998')

But ROUND_DOWN result of 1.9999999999999998 with 0.000001 precision should be 1.999999 thus py2 behaviour is wrong.