Use VlanId.vlanId(short) to validate the value

Also update the unit test
    - VlanId.vlanId("5000") should throw IllegalArgumentException

Change-Id: I374532080dc2a51da8f2c0a46ea92df6f8abbe56

@@ -92,7 +92,7 @@ public final class VlanId extends Identifier<Short> {
             return new VlanId(ANY_VALUE);
         }
         try {
-            return new VlanId(Short.parseShort(value));
+            return VlanId.vlanId(Short.parseShort(value));
         } catch (NumberFormatException e) {
             throw new IllegalArgumentException(e);
         }

@@ -25,7 +25,6 @@ public class VlanIdTest {
 
     @Test
     public void testEquality() {
-
         VlanId vlan1 = VlanId.vlanId("None");
         VlanId vlan2 = VlanId.vlanId((short) -1);
         VlanId vlan3 = VlanId.vlanId((short) 100);
@@ -48,11 +47,15 @@ public class VlanIdTest {
         assertEquals("incorrect VLAN value", 10, vlan1.toShort());
         assertEquals("invalid untagged value", VlanId.UNTAGGED,
                      vlan2.toShort(), vlan3.toShort());
-
     }
 
     @Test(expected = IllegalArgumentException.class)
-    public void testIllicitVLAN() {
+    public void testIllicitVlan() {
         VlanId.vlanId((short) 5000);
     }
+
+    @Test(expected = IllegalArgumentException.class)
+    public void testIllicitVlanString() {
+        VlanId.vlanId("5000");
+    }
 }