add support for resetting transient + persistent cluster settings

This PR implements support for resetting transient & persistent cluster settings to, either the configuration file value defined at node startup, or if it does not existed, the default value.

Resetting settings is done using the normal cluster update settings API, by just assigning a JSON `null` to the setting one want to reset. See `update-settings.asciidoc` for details.

There are maybe other ways to do that, but I think this is just straight-forward and it works ;)
This is related to https://github.com/elasticsearch/elasticsearch/issues/6732, but does only apply to cluster settings not custom analyzer.