[Feature request] Add a Elasticsearch cluster settings methods

[Oloremo]

Description of Issue

Please add a get_settings and put_settings methods from the Python elasticsearch library: https://elasticsearch-py.readthedocs.io/en/master/api.html

Also, the flush_synced API call is good to have.

Having these methods are critical for the implementation of a rolling update logic: https://www.elastic.co/guide/en/elasticsearch/reference/current/rolling-upgrades.html