[fix] Allow updating templates with invalid configurations

Previously, fixing an invalid template configuration via the UI was
blocked due to the cache invalidation mechanism. This mechanism
attempted to evaluate the existing configuration, triggering a
ValidationError and preventing updates.

(cherry picked from commit bd3d11a)

openwisp_controller/config/tests/test_template.py

@@ -748,3 +748,18 @@ def test_task_timeout(self, mocked_update_related_config_status):
             template.save()
             mocked_error.assert_called_once()
         mocked_update_related_config_status.assert_called_once()
+
+    def test_fixing_wrong_configuration(self):
+        template = self._create_template()
+        # create a wrong configuration
+        Template.objects.update(config={'interfaces': [{'name': 'eth0', 'type': ''}]})
+        # Ensure the configuration raises ValidationError
+        with self.assertRaises(NetjsonconfigValidationError):
+            template.refresh_from_db()
+            del template.backend_instance
+            template.checksum
+
+        del template.backend_instance
+        template.config = {'interfaces': [{'name': 'eth0', 'type': 'ethernet'}]}
+        template.full_clean()
+        template.save()