[ONOS-5400][ONOS-5614] identity ref support in yangtools and defect fixes.

Change-Id: Ia1f32a6772e9d468717340953cf7598c61d9f1ed
diff --git a/generator/src/main/java/org/onosproject/yangutils/utils/UtilConstants.java b/generator/src/main/java/org/onosproject/yangutils/utils/UtilConstants.java
index 025c8fc..c289e77 100644
--- a/generator/src/main/java/org/onosproject/yangutils/utils/UtilConstants.java
+++ b/generator/src/main/java/org/onosproject/yangutils/utils/UtilConstants.java
@@ -228,6 +228,16 @@
     public static final String PERIOD = ".";
 
     /**
+     * Static attribute for period.
+     */
+    public static final String ENTRY = "Entry";
+
+    /**
+     * Static attribute for period.
+     */
+    public static final String ENTRY_SET = "entrySet()";
+
+    /**
      * Static attribute for compare to.
      */
     public static final String COMPARE_TO = "compareTo";
@@ -621,6 +631,31 @@
     public static final String ADD_STRING = "add";
 
     /**
+     * Static attribute for key syntax.
+     */
+    public static final String KEYS = "Keys";
+
+    /**
+     * Static attribute for key syntax.
+     */
+    public static final String GET_KEY = "getKey()";
+
+    /**
+     * Static attribute for key syntax.
+     */
+    public static final String GET_VALUE = "getValue()";
+
+    /**
+     * Static attribute for -1 to syntax.
+     */
+    public static final String NEG_ONE = "-1";
+
+    /**
+     * Static attribute for Comparable to syntax.
+     */
+    public static final String COMPARABLE = "Comparable";
+
+    /**
      * Static attribute for string trim syntax.
      */
     public static final String TRIM_STRING = "trim";
@@ -676,6 +711,11 @@
     public static final String VALUE = "value";
 
     /**
+     * Static attribute for value syntax.
+     */
+    public static final String VALUE_CAPS = "Value";
+
+    /**
      * Static attribute for suffix s.
      */
     public static final String SUFFIX_S = "s";
@@ -890,8 +930,23 @@
     /**
      * Static attribute for exception string in bits enum class.
      */
-    public static final String EXCEPTION_STRING = "IllegalArgumentException(\"no" +
-            " such element found in bits\");\n";
+    public static final String EXCEPTION_STRING = "IllegalArgumentException" +
+            "(\"not a valid input element\");\n";
+
+    /**
+     * Static attribute for priority queue.
+     */
+    public static final String PRIORITY_QUEUE = "java.util.PriorityQueue<>()";
+
+    /**
+     * Static attribute for linked hash set.
+     */
+    public static final String LINKED_HASH_SET = "java.util.LinkedHashSet<>()";
+
+    /**
+     * Static attribute for priority queue.
+     */
+    public static final String LINKED_HASH_MAP = "java.util.LinkedHashMap<>()";
 
     /**
      * Static attribute for "throw new ".