You might get an answer if you go through this.
I guess you have misunderstood the concept of ROS_MASTER. It is true that roscore is the master (in robotless scenario) but not in the sense you have understood and other nodes running are not slave nodes. roscore is not a single node, it is a collection of nodes needed to run a ROS system. Master node is the one that keeps track of other nodes, services, topics etc. Read more here and here. Of course you can write nodes for swarm robotics where one node acts as a master and others are slaves but relationship between roscore and other nodes is not the proper analogy.