[LMS-2222]: minor installer improvements (added an empty jython dropbox)

screening/build/build.xml

screening/dist/etc/service.properties

screening/dist/installer/dropboxes/simple-dropbox.py

screening/dist/installer/incoming-simple/.gitignore

screening/dist/installer/install.xml

screening/dist/installer/sample-datasets/.gitignore

screening/dist/installer/userInputSpec.xml

screening/dist/server/service.properties

.../cisd/openbis/installer/izpack/DBConnectionValidator.java