string issue fix for few yang construct like config, mandatory + allowing yang constructs as identifers and string

Change-Id: I8dd01dc60d280a843b0a485681caa6bfcac013c2
diff --git a/utils/yangutils/src/test/java/org/onosproject/yangutils/parser/impl/listeners/ConfigListenerTest.java b/utils/yangutils/src/test/java/org/onosproject/yangutils/parser/impl/listeners/ConfigListenerTest.java
index fe20cad..7e693d5 100644
--- a/utils/yangutils/src/test/java/org/onosproject/yangutils/parser/impl/listeners/ConfigListenerTest.java
+++ b/utils/yangutils/src/test/java/org/onosproject/yangutils/parser/impl/listeners/ConfigListenerTest.java
@@ -115,7 +115,7 @@
     @Test
     public void processConfigInvalidValue() throws IOException, ParserException {
         thrown.expect(ParserException.class);
-        thrown.expectMessage("mismatched input 'invalid' expecting {'false', 'true'}");
+        thrown.expectMessage("YANG file error : config value invalid is not valid.");
         YangNode node = manager.getDataModel("src/test/resources/ConfigInvalidValue.yang");
     }
 
@@ -125,7 +125,7 @@
     @Test
     public void processConfigEmptyValue() throws IOException, ParserException {
         thrown.expect(ParserException.class);
-        thrown.expectMessage("missing {'false', 'true'} at ';'");
+        thrown.expectMessage("no viable alternative at input ';'");
         YangNode node = manager.getDataModel("src/test/resources/ConfigEmptyValue.yang");
     }