Notifications widget: support custom interval

If 'interval' attribute is passed in notification object, use that
interval instead of the default.

ui/scripts/ui/widgets/notifications.js

@@ -276,7 +276,7 @@
         cloudStack.ui.event.call('addNotification', {
           section: notification.section,
           desc: notification.desc,
-          interval: 5000,
+          interval: notification.interval ? notification.interval : 5000,
           _custom: notification._custom,
           poll: function(args) {
             var complete = args.complete;