Instance wizard: show loading overlay on launch

When clicking the launch VM button, immediately show a loading overlay,
preventing multple clicks which can accidentally launch multiple VMs.

reviewed-by: jessica

ui/scripts/ui-custom/instanceWizard.js

@@ -22,6 +22,7 @@
         // Save instance and close wizard
         var completeAction = function() {
           var data = cloudStack.serializeForm($form);
+          var $wizardLoading = $('<div>').addClass('loading-overlay').appendTo($wizard).css('z-index', '10000');
 
           args.action({
             // Populate data