scaleutils: add post scale helper to run node purge after scale in

[jrasell]

The Nomad node purge API call was recently added to the Nomad Go SDK. This addition allows the Nomad Autoscaler to purge Nomad nodes once they have been scaled in. This keeps the node list tidy particuary in environments which scale on a frequent basis. Without the purge call, the nodes will eventually be removed from the list by the Nomad GC.

The behaviour should probably have a config flag, allowing the operator to decide whether or not scaled nodes are purge from the system.