Add endpoint for dumping all effective settings

[drewr]

We should provide an endpoint that dumps a map of all settings and their values.

    % curl localhost:9200/_config
    {
        "cluster": {
            "name": "elasticsearch"
        }, 
        "index": {
            "number_of_shards": 1
        }, 
        "node": {
            "data": true, 
            "master": true
        },
        "etc": "..."
    }

[lmenezes]

is this getting any attention at all?
if not, I will be glad to work on it :)

[clintongormley]

Closed in favour of #6732