When sending an update settings request to /_cluster/settings with minimum_master_nodes I get the following log entry on my master nodes. It also does not apply the settings.
[2015-05-16 21:49:54,425][WARN ][action.admin.cluster.settings] [i-148083f2] ignoring persistent setting [discovery.zen.minimum_master_nodes], not dynamically updateable
I've searched all over mailing lists and asked on IRC, but I really cannot tell why this doesn't work. I'm using 1.5.1 on ubuntu and I am using the EC2 discovery plugin, if that matters.