Don't write default value to configuration file when default value came from /etc/* file
Summary: This will allow administrator to change default value and be reflected at user level
Depends on D28128.
Test Plan: added an unit test
Reviewers: ervin, dfaure, davidedmundson
Subscribers: kde-frameworks-devel
Tags: #frameworks
Differential Revision: https://phabricator.kde.org/D28221