ASG warm pool support

[VSilantyev]

The idea behind this PR is to allow ASG to use warm pool, for faster builds - it's faster to get stopped instance from warm pool, than spin up it from image, and stopped instance will have caches/checked out code from last build.

Implementation details:
Instead of terminating instance - plugin removes scale-in protection from the instance it wants to terminate and allows ASG to handle instance termination - AGS will move this instance back to warm pool if there is one configured.

Related to #352

Testing done

Tested on live jenkins instance, and corrected automated tests.

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[BradyShober]

This change breaks the use case of configuring a maximum number of builds. When an instance reaches the maximum number of builds, the desired capacity may not change, but the plugin will set the instance offline within Jenkins then execute the termination. If the plugin is changed to no longer issue terminations, then those instances would never be replaced and just sit idle not able to process any jobs.

[VSilantyev]

Added logic - terminate if max uses exhausted, remove scale in protection otherwise.