ParameterValueDescriptor: default and sample values are optional

Change-Id: Ie693f405d3f46488a8869a29b52d58bf866b7b90
Reviewed-on: http://gerrit.esdi-community.eu/3125
Tested-by: Jenkins <sitemple@igd.fraunhofer.de>
Reviewed-by: Simon Templer <simon.templer@igd.fraunhofer.de>