Fix the default tax bug lp:796570.

Fix the default tax bug https://code.launchpad.net/bugs/796570

As requested by Fabien (https://bugs.launchpad.net/openobject-addons/+bug/796570/comments/5) the bug have been corrected on all the modules where the default tax is ignored.

Some functions have been added on the module 'product_product' (get_supplier_taxes_ids and get_taxes_ids). Those functions are now used in the corrected modules and in account, hr_expense and hr_timesheet_invoice in order to have the same logic everywhere.
in v8, it may still be time to pass the context here and in the entry point  method. But ok for v7