[FIX] Use the migration machinery to initialize default values

backported from https://code.launchpad.net/~acsone-openerp/account-financial-report/7.0-bug-1312732-lmi/+merge/217240

The proposal is valid for the 7.0 since the migration script is no more called on fresh install in 8.0. For 8.0 the migration script can be replaced by an install hook (https://github.com/odoo/odoo/commit/4105b5f0285821d8056c721b5fd50b9cae902988)