STYLE: dashboarddriver: Update macro setOnlyIfNotDefined to display info

This commit updates the macro "setOnlyIfNotDefined" to display the
environment variable value (introduced in r23417) if it has been set.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23418 3bd1e089-480b-0410-8dfb-8563597acbee