read default Qt version name from the configuration
read default Qt version name from the configuration

The default Qt version name can be set with
    qbs config --global defaults/qtVersionName qt5

Change-Id: I8c046d2bdac49d15c89f780250ca94f851d4c3e3
Reviewed-by: Oliver Wolff <bfa2c373f7b4cae624ec1928f54bc38b8cd9f442@nokia.com>

villytiger/qbs,gatzka/qt-labs-qbs,villytiger/qbs,Distrotech/qbs,qt-labs/qbs,Distrotech/qbs,Distrotech/qbs,qtproject/qt-labs-qbs,Distrotech/qbs,Distrotech/qbs,villytiger/qbs,gatzka/qt-labs-qbs,qtproject/qt-labs-qbs,gatzka/qt-labs-qbs,Distrotech/qbs,qtproject/qt-labs-qbs,qtproject/qt-labs-qbs,gatzka/qt-labs-qbs,qt-labs/qbs,qtproject/qt-labs-qbs,gatzka/qt-labs-qbs,gatzka/qt-labs-qbs,qt-labs/qbs,villytiger/qbs,qt-labs/qbs,villytiger/qbs,villytiger/qbs,qt-labs/qbs,gatzka/qt-labs-qbs,Distrotech/qbs,qtproject/qt-labs-qbs,qt-labs/qbs,qtproject/qt-labs-qbs,qt-labs/qbs