Ensure that removing the oldest master node doesn't leave the cluster with less than 3 master-eligible nodes

[jacobwinch]

According to the ES docs, it's safe to (slowly) remove master nodes:

As long as there are at least three master-eligible nodes in the cluster

At the moment it's possible for the Step Function to create a situation where we have less than 3 master-eligible nodes in the cluster. Although this is unlikely to happen (it would only occur if something/someone else made master-eligible node(s) unavailable whilst the Step Function was running), it would still be good to enforce this check just before removing a master node (e.g. here).