Enhancement: Reassign Instance to a Different ISO or Template

### The required feature described as a wish

As a User I would like to unassign and reassign an instance to another ISO or template. It's not possible to guarantee a template will always be around and it's a waste of space to keep older templates. We have a need to update templates on a periodic basis.

该 issue 未提及任何文件、测试或入口点，也未定义重新分配行为或验收标准。首先追踪 CloudStack 如何将实例与 ISO 或模板关联，并确定相关的 API 和测试。在实现之前，确认是否同时需要支持 ISO 和模板的重新分配，并定义对于现有实例而言“完成”意味着什么。

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