Fix indexing into subnet list during cluster creation

When creating a cluster using an existing network,
once the network has been selected from the dropdown
the subnet dropdown fails to populate. This is due to
an off-by-one indexing error.

Closes-Bug: #2087843

Change-Id: I0f7b20576e25c9f93ddc90fcfb501422d2adb7e1

magnum_ui/static/dashboard/container-infra/clusters/workflow/workflow.service.js

@@ -686,7 +686,7 @@
           fixedSubnets = [{value: "", name: fixedSubnetsInitial}];
         }
         // NOTE(dalees): This hardcoded index could be improved by referencing an object instead.
-        form[0].tabs[2].items[0].items[0].items[3].titleMap = subnetTitleMap;
+        form[0].tabs[2].items[0].items[0].items[2].titleMap = subnetTitleMap;
         model.fixed_subnet = MODEL_DEFAULTS.fixed_subnet;
       }