CS-15740: Show 'please select a tier' instead of blank default option

For VPC enable static NAT dialog, a tier always needs to be selected
-- this is now indicated in the default option select.

ui/scripts/network.js

@@ -1387,7 +1387,7 @@
                           },
                           success: function(json) {
                             var networks = json.listnetworksresponse.network;
-                            var items = [{ id: -1, description: '' }];
+                            var items = [{ id: -1, description: 'Please select a tier' }];
                             $(networks).each(function(){
                               items.push({id: this.id, description: this.displaytext});
                             });