Response for Cluster Settings Update API

If cluster settings are update the REST API returns the accepted values. For
example, updating the `cluster.routing.allocation.disable_allocation` via
cluster settings:

```
curl -XPUT http://localhost:9200/_cluster/settings -d '{
    "transient":{
        "cluster.routing.allocation.disable_allocation":"true"
    }
}'
```

will respond:

```
{
    "persistent":{},
    "transient":{
        "cluster.routing.allocation.disable_allocation":"true"
    }
}
```

Closes #2907