InspireInstanceWriter: add identifier as constant

Change-Id: I125663f8e4eb26d617df48389591c266159eae7f
Reviewed-on: http://gerrit.esdi-community.eu/3243
Tested-by: Jenkins <sitemple@igd.fraunhofer.de>
Reviewed-by: Simon Templer <simon.templer@igd.fraunhofer.de>