Add list node's identifier info to toString

Change-Id: I79038abb827ab95afea1c183dd3f0b4bdf73d816
diff --git a/model/src/main/java/org/onosproject/yang/model/ListKey.java b/model/src/main/java/org/onosproject/yang/model/ListKey.java
index 05eab8c..b5be8f7 100644
--- a/model/src/main/java/org/onosproject/yang/model/ListKey.java
+++ b/model/src/main/java/org/onosproject/yang/model/ListKey.java
@@ -103,6 +103,7 @@
     @Override
     public String toString() {
         return toStringHelper(getClass())
+                .add("identifier", schemaId())
                 .add("value", keyLeafs)
                 .toString();
     }