Configuration models can read selection values from target model
Currently when we declare the configuration of a selection field, we redefine the possible values and this causes all the modules which extends it to modify the value on two places. It will be great if the selection values can be directly read from the target model like it's done on product.cost_method [1], so the selection will be only declared once and this will simplify it's management.
[1] http://hg.tryton.org/modules/product/file/tip/configuration.py#l29