bug 839: fix error looking for nsi using a nst

Change-Id: Iba0af5eccc16255eb408531fd5b0da3637651274
Signed-off-by: tierno <alfonso.tiernosepulveda@telefonica.com>
diff --git a/osm_nbi/descriptor_topics.py b/osm_nbi/descriptor_topics.py
index 27b51a1..0c9a3a4 100644
--- a/osm_nbi/descriptor_topics.py
+++ b/osm_nbi/descriptor_topics.py
@@ -854,7 +854,7 @@
             return
         # Get Network Slice Template from Database
         _filter = self._get_project_filter(session)
-        _filter["nst-id"] = _id
+        _filter["_admin.nst-id"] = _id
         if self.db.get_list("nsis", _filter):
             raise EngineException("there is at least one Netslice Instance using this descriptor",
                                   http_code=HTTPStatus.CONFLICT)