Add toString for ease of debugging

Change-Id: I8974be26573713618facda6b3a80a3dc970db2bd
diff --git a/model/src/main/java/org/onosproject/yang/model/InnerNode.java b/model/src/main/java/org/onosproject/yang/model/InnerNode.java
index 35b56b8..7d07056 100644
--- a/model/src/main/java/org/onosproject/yang/model/InnerNode.java
+++ b/model/src/main/java/org/onosproject/yang/model/InnerNode.java
@@ -52,6 +52,14 @@
         childNodes = builder.childNodes;
     }
 
+    @Override
+    public String toString() {
+        return "{" +
+                    "key=" + key() + ", " +
+                    "childNodes=" + childNodes().values() +
+               "}";
+    }
+
     /**
      * Returns inner node builder instance.
      *
diff --git a/model/src/main/java/org/onosproject/yang/model/LeafNode.java b/model/src/main/java/org/onosproject/yang/model/LeafNode.java
index 9149238..d66cb10 100644
--- a/model/src/main/java/org/onosproject/yang/model/LeafNode.java
+++ b/model/src/main/java/org/onosproject/yang/model/LeafNode.java
@@ -47,6 +47,14 @@
         return String.valueOf(value);
     }
 
+    @Override
+    public String toString() {
+        return "{" +
+                    "key=" + key() + ", " +
+                    "value=" + asString() +
+               "}";
+    }
+
     /**
      * Creates an instance of leaf node.
      *