config ang key validation and UT for the same

Change-Id: I507740fc9da3f3da5fb3c88a7414f87db6251c5b
diff --git a/utils/yangutils/src/main/java/org/onosproject/yangutils/datamodel/YangLeaf.java b/utils/yangutils/src/main/java/org/onosproject/yangutils/datamodel/YangLeaf.java
index 764f498..888094e 100644
--- a/utils/yangutils/src/main/java/org/onosproject/yangutils/datamodel/YangLeaf.java
+++ b/utils/yangutils/src/main/java/org/onosproject/yangutils/datamodel/YangLeaf.java
@@ -67,7 +67,7 @@
     /**
      * If the leaf is a config parameter.
      */
-    private boolean isConfig;
+    private Boolean isConfig;
 
     /**
      * description of leaf.
@@ -128,7 +128,7 @@
      *
      * @return if config flag.
      */
-    public boolean isConfig() {
+    public Boolean isConfig() {
         return isConfig;
     }