Fix types field in JSON Search Slow Logs

[pgomulka]

The field has to be defined in log4j2.properties and should be an
escaped JSON for now (it is a broken JSON at the moment). This should later be refactored into a JSON array
of strings.
"types": "[\"type1\", \"type2\"]",

{
  "type": "index_search_slowlog",
  "timestamp": "2019-07-22T10:36:07,942+02:00",
  "level": "WARN",
  "component": "i.s.s.query",
  "cluster.name": "distribution_run",
  "node.name": "node-0",
  "message": "[index6][0]",
  "took": "8.9ms",
  "took_millis": "8",
  "total_hits": "0 hits",
  "types": "[\"type1\", \"type2\"]",
  "stats": "[]",
  "search_type": "QUERY_THEN_FETCH",
  "total_shards": "1",
  "source": "{}",
  "id": "ASDF",
  "cluster.uuid": "PkkHdglrTiiWfIE_0V4xWA",
  "node.id": "LFhU0lQaRNmxQPewPk0l7w"
}

[elasticmachine]

Pinging @elastic/es-core-infra

[alpar-t]

LGTM