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.

首先追踪主机禁用和进入维护模式的 UI 与 API 路径。确认当前处于 Up 和 Disabled 状态的主机是如何处理的，然后验证它可以进入维护模式而不会被重新启用，并且维护流程会避开 issue 中描述的调度时间窗口。

由索引模型根据 Issue 内容生成。