Allow `repl` config option passed into clusterMaster constructor
* `repl` - where to have REPL listen, defaults to `env.CLUSTER_MASTER_REPL` || 'cluster-master-socket'
* if `repl` is null or false - REPL is disabled and will not be started
* if `repl` is string path - REPL will listen on unix domain socket to this path
* if `repl` is an integer port - REPL will listen on TCP 0.0.0.0:port
* if `repl` is an object with `address` and `port`, then REPL will listen on TCP address:PORT
Since quit is a common command to exit certain
types of command shells, I felt it would be better
to use a different name for stopping the cluster,
since it would be easy to get confused and accidentally
shut down the cluster when simply wanting to exit the
I switched the REPL command to stop() for a graceful
shutdown and a kill() for stopping hard.