elasticsearch: support changing minimumMasters field

[munnerz]

We need to work out and implement the best practice method for changing the minimumMasters field.

Until we do, it isn't possible to safely scale a node pool of Elasticsearch masters up the old masters would still retain the old config.

We may want to do this through the Elasticsearch config API. The 'leading' pilot could be responsible for applying the configuration change to the cluster.

/kind feature
/area elasticsearch