Generalize member-vnf-index as string in constituent-vnfd in NSD to allow symbolic names

Change-Id: Iddfdf664bab686a6da1e50b313b1131783e71078
Signed-off-by: garciadeblas <gerardo.garciadeblas@telefonica.com>
diff --git a/models/yang/nsd.yang b/models/yang/nsd.yang
index 55b40eb..19aebe9 100644
--- a/models/yang/nsd.yang
+++ b/models/yang/nsd.yang
@@ -65,7 +65,7 @@
           "Identifier/index for the VNFD. This separate id
            is required to ensure that multiple VNFs can be
            part of single NS";
-        type uint64;
+        type string;
       }
 
       leaf vnfd-id-ref {