[REF] purchase: use config parameter instead of ir values in settings

Fields like `default_foo` still used ir.values although those are system-wide
parameters and should therefore be stored in ir.config_parameter
table.

When no default_model is set on a field, don't prefix it by `default_`.

Also, reset po_lead when use_po_lead is unchecked to avoid it to be used it further computations.

addons/purchase/models/stock_config_settings.py

@@ -8,4 +8,20 @@ class StockConfigSettings(models.TransientModel):
     _inherit = 'stock.config.settings'
 
     po_lead = fields.Float(related='company_id.po_lead', default=lambda self: self.env.user.company_id.po_lead)
-    default_new_po_lead = fields.Boolean(string="Security Lead Time for Purchase", default_model="stock.config.settings", help="Margin of error for vendor lead times. When the system generates Purchase Orders for reordering products,they will be scheduled that many days earlier to cope with unexpected vendor delays.")
+    use_po_lead = fields.Boolean(
+        string="Security Lead Time for Purchase",
+        oldname='default_new_po_lead',
+        help="Margin of error for vendor lead times. When the system generates Purchase Orders for reordering products,they will be scheduled that many days earlier to cope with unexpected vendor delays.")
+
+    @api.onchange('use_po_lead')
+    def _onchange_use_po_lead(self):
+        if not self.use_po_lead:
+            self.po_lead = 0.0
+
+    def get_default_fields(self, fields):
+        return dict(
+            use_po_lead=self.env['ir.config_parameter'].sudo().get_param('purchase.use_po_lead')
+        )
+
+    def set_fields(self):
+        self.env['ir.config_parameter'].sudo().set_param('purchase.use_po_lead', self.use_po_lead)

addons/purchase/views/stock_config_settings_views.xml

@@ -9,16 +9,16 @@
                 <div id="purchase_po_lead" position="replace">
                     <div class="col-xs-12 col-md-6 o_setting_box" title="Margin of error for vendor lead times. When the system generates Purchase Orders for reordering products,they will be scheduled that many days earlier to cope with unexpected vendor delays.">
                         <div class="o_setting_left_pane">
-                            <field name="default_new_po_lead"/>
+                            <field name="use_po_lead"/>
                         </div>
                         <div class="o_setting_right_pane">
-                            <label for="default_new_po_lead"/>
+                            <label for="use_po_lead"/>
                             <span class="fa fa-lg fa-building-o" title="Values set here are company-specific." groups="base.group_multi_company"/>
                             <div class="text-muted">
                                 Schedule receivings earlier to avoid delays
                             </div>
                             <div class="content-group">
-                                <div class="mt16" attrs="{'invisible': [('default_new_po_lead','=',False)]}">
+                                <div class="mt16" attrs="{'invisible': [('use_po_lead','=',False)]}">
                                     <span>Move forward expected delivery dates by <field name="po_lead" class="oe_inline"/> days</span>
                                 </div>
                             </div>