Switch integration-jjb-config-diff to Nodepool
Change label to ci-jessie-wikimedia, that designates the instances
spawned by Nodepool.
Have start/end of build to emit events over ZeroMQ to notify Nodepool.
Let it knows that node is being used / can be deleted.
Apply the 'offline_when_complete' parameter function to inject in the
Zuul gearman protocol NODE_OFFLINE_WHEN_COMPLETE=1 which causes the
Jenkins gearman plugin to put the node offline, preventing further build
allocation to that node by Jenkins.