Update Settings: Allow to control index.auto_expand_replicas

[kimchy]

Allow to change the index.auto_expand_replicas setting dynamically using update setting. Also, allow it to be set to false, basically, disabling it. For example:

curl -XPUT localhost:9200/test -d '{
    "settings" : {
        "index" : {
            "number_of_shards" : 1,
            "auto_expand_replicas" : "0-all"
        }
    }
}'

curl -XPUT localhost:9200/test/_settings -d '{
    "index" : {
        "number_of_replicas" : 0,
        "auto_expand_replicas" : false
    }
}'

curl -XPUT localhost:9200/test/_settings -d '{
    "index" : {
        "auto_expand_replicas" : "0-all"
    }
}'

[kimchy]

Update Settings: Allow to control index.auto_expand_replicas, closed by 8f8fb89.