jenkins: Limit number of builds scheduled in jenkins

Jenkins will 'forgot' or 'lose' builds if the queue size grows too large, and it can be difficult to clean up a failed job build when the queue is full of leeroy jobs triggered by it, so this limits the number of builds in the queue at any one time to no more than 10.

Note that builds for rebooting hosts or cleaning workspaces are ignored, this only considered builds not related to those tasks.