compass ci's zmq publisher on jenkins died causing nodepool not to delete used slaves

[Jerry Zhao]

The zeromq event publisher died although connection to 8888 port was still there, so nodepool wouldn't set node as used if a job ran on it. Moreover, zuul's layout.yaml didn't have singe_used_node configured for compass-dev-demo job, so when the job compass-dev-demo job ran on nodepool nodes, the slave wouldn't be cleaned after it was used and status still remained 'ready'. Later new jobs would still ran on those 'used' nodes.