Allow Host to be put in Maintenance Mode while Up and Disabled

##### ISSUE TYPE

* Improvement Request

##### COMPONENT NAME

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UI, API
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##### CLOUDSTACK VERSION

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4.18.1.0
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##### SUMMARY

Ability to put host in maintenance mode when host is Up and Disabled. Disabling the host prevents new loads from being added when performing maintenance on groups of host. This prevents loads from being repeatedly moved. Requiring the host to be re-enabled prior to putting in maintenance mode opens a window for a VM to be scheduled for the host. Simplifies and improves the maintenance process.

首先追蹤主機停用和進入維護模式的 UI 與 API 路徑。確認目前處於 Up 和 Disabled 狀態的主機是如何處理的，然後驗證它可以進入維護模式而不會被重新啟用，且維護流程會避開 issue 中描述的排程時間窗口。

由索引模型根據 Issue 內容生成。