Allow location of queue.xml to be overridden

[jglick]

Allows the location of the queue persistence file for the controller to be overridden on a per-session basis. This is required for functionality in CloudBees CI but I do not foresee any use case in OSS Jenkins, so it should not be documented for Jenkins administrators.

Testing done

Tested in the context of CloudBees CI.

Proposed changelog entries

• N/A

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[MarkEWaite]

This PR is now ready for merge. We will merge it after approximately 24 hours if there is no negative feedback.

[MarkEWaite]

/label ready-for-merge