Unregister on 'stop'

https://github.com/gliderlabs/registrator/blob/master/registrator.go#L180-L184 indicates Registrator deregisters when a container exits.  Clients of the service will have already received an error at that point.  It'd be more graceful to mark the instance of the service as in maintenance mode concurrent with the 'stop' phase.

https://www.consul.io/api/agent/service.html#sample-request-4
