[ADD] partner_firstname for v8

because it's working as is
After reading the code in openerp.models, you're right, the set_default_value_on_column is the right way to do it since it's only called on  the first installation or when upgrading the module 'ONLY' if the 'Not null' constrains is not yet applied. In proposing the 'install hooks', I wanted to avoid that this operation (_set_default_value_on_calumn) is called each time the module is upgraded.