Discovery: don't accept a dynamic update to min_master_nodes which s larger then current master node count

The discovery.zen.minimum_master_nodes setting can be updated dynamically. Settings it to a value higher then the current number of master nodes will cause the current master to step down. This is dangerous because if done by mistake (typo) there is no way to restore the settings (this requires an active master).
the test used to set the min_master_node to 2 in order to cause the single node to step down from being a master. This is not possible any more.