[FIX] sale: unable to install module sale

When you have another module installed before sale, and ever changed
some settings, a record of res_config_settings exists and when you
install sale, it cannot set a value for the required new column
'sale_show_tax' for existing records.

To fix this, we have set back the default values on the required field
'sale_show_tax' that has been removed in commit: https://github.com/odoo/odoo/commit/71e5c8b0b5bb10f571030a25875e33b25cda073d

addons/sale/models/res_config_settings.py

@@ -47,7 +47,7 @@ class ResConfigSettings(models.TransientModel):
     sale_show_tax = fields.Selection([
         ('subtotal', 'Tax-Excluded Prices'),
         ('total', 'Tax-Included Prices')], string="Tax Display",
-        required=True)
+        default='subtotal', required=True)
     default_invoice_policy = fields.Selection([
         ('order', 'Invoice what is ordered'),
         ('delivery', 'Invoice what is delivered')