Calculate IGMP checksum and use more reasonble max response time.

Also made IGMP properties configurable at runtime.

Change-Id: I98b40a43a0c17b7bf21f1bd622032c64d7434214
diff --git a/core/api/src/main/java/org/onosproject/cfg/ConfigProperty.java b/core/api/src/main/java/org/onosproject/cfg/ConfigProperty.java
index f1b602e..8d0bce8 100644
--- a/core/api/src/main/java/org/onosproject/cfg/ConfigProperty.java
+++ b/core/api/src/main/java/org/onosproject/cfg/ConfigProperty.java
@@ -44,6 +44,11 @@
         STRING,
 
         /**
+         * Indicates the value is a byte.
+         */
+        BYTE,
+
+        /**
          * Indicates the value is an integer.
          */
         INTEGER,
@@ -194,6 +199,16 @@
     }
 
     /**
+     * Returns the property value as a byte.
+     *
+     * @return byte value
+     */
+    public byte asByte() {
+        checkState(type == Type.BYTE, "Value is not a byte");
+        return Byte.parseByte(value);
+    }
+
+    /**
      * Returns the property value as an integer.
      *
      * @return integer value