server_environment: Fix value assignation in compute from default

Since previous commit adding support for ssl certificate auth,
field smtp_authentication in ir.mail_server, became a field managed by
environment, when mail_environment is installed.

This field has been defined using a default value relying on the
compute_default property allowing to define a compute function,
which matches the default value of the field.

However, as the function _compute_server_env_from_default was broken,
because it did not assign the computed value to the field, the
field was empty and connection was broken.

server_environment/models/server_env_mixin.py

@@ -254,7 +254,7 @@ def _compute_server_env_from_config(self, field_name, options):
 
     def _compute_server_env_from_default(self, field_name, options):
         if options and options.get("compute_default"):
-            getattr(self, options["compute_default"])()
+            self[field_name] = getattr(self, options["compute_default"])()
         else:
             default_field = self._server_env_default_fieldname(field_name)
             if default_field: