[Feature Request] Add Default PreviousLevel property to OK on AlertNode

When configuring AlertNode with stateChangesOnly I've notice that alerts will be sent only on State Changes but once PreviousLevel has been set at least once.

This is a problem when you begins to work with some devices already in the CRITICAL or WARNING state. In this scenario we need alert() sends notification for these devices in the first condition check.

Right now alertNode waits until these devices changes state to send it's first notification , and this happens after the problem has been fixed. ( some hours after) .

IMHO this behaviour is not expected. Suppose you change your monitoring system on 100 devices with 20 of them in Critical state. On the first deploy of the task won't sent notifications for these 20 devices, and customer could think this new monitoring system is not working ok.

Start by locating the AlertNode implementation and the handling of stateChangesOnly, PreviousLevel, and alert(). Verify the first condition check for devices already in WARNING or CRITICAL state, then add coverage showing that an initial notification is sent and subsequent unchanged states remain suppressed.

Written by the indexing model from the issue text.