UI: Assign Instance Group to Domain/Account during Creation

### The required feature described as a wish

As an Admin or Operator, I would like to be able to Instance Group to Domain/Account in the UI. Currently the only way to do this is by logging into a user in account that you want the instance group to be created in.

首先定位用於建立 Instance Group 的 UI 入口，並檢查目前如何選取 Domain 或 Account 情境。在定義變更之前，確認現有的建立流程和權限。完成的標準是，Admin 或 Operator 可以在建立期間將 Instance Group 指派給 Domain 或 Account，而不需要登入該 account。

由索引模型根據 Issue 內容生成。