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).
I had commit on my machine I forget to push changing this back to it's original value - 10