Allow to pass arbitrary request parameters for ES queries

[danielmitterdorfer]

We should allow users to pass arbitrary request parameters for queries (i.e. operation-type search in Rally). The syntax is best shown by an example (field request-params):

    {
      "name": "default",
      "operation-type": "search",
      "body": {
        "query": {
          "match_all": {}
        }
      },
      "request-params": {
        "_source_include": "geonameid",
        "analyze_wildcard": false
      }
    }

The supported parameters names are documented in the Python ES client API docs. Parameters that are implicitly set by Rally (e.g. body or request_cache) are not supported (i.e. users are not allowed to use them and if they do so behavior is unspecified).