[API] Added the `flat_settings` parameter to "Get Cluster Settings" API

Related: elasticsearch/elasticsearch#4140, elasticsearch/elasticsearch#4497
[API] Added missing parameters for the "Get Index Settings" and "Put Index Settings" APIs

* prefix
* ignore_indices
* ignore_unavailable
* allow_no_indices
* expand_wildcards
* flat_settings

Related: elasticsearch/elasticsearch#4436, elasticsearch/elasticsearch#4453