Replace usage of SlaveBase::config()->readEntry by SlaveBase::configValue

Summary:
Followup after D24582

There are only 5 usage of SlaveBase::config() that are more intrusive to replace

Reviewers: dfaure

Reviewed By: dfaure

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D24685