Multiple auth dialogs when using Basic Auth

[spalger]

Kibana version: master
Elasticsearch version: master
Original install method (e.g. download page, yum, from source, etc.): source

Code in the elasticsearch plugin is trying to parse the error messages elasticsearch produces when it requires basic authentication, in order to determine the correct realm. As the // TODO: indicates, this was intended as a stop-gap until the header was available from esjs.

ES has since changed the format of the error messages, so the header is no longer included, but it's now accessible in the response body:

{
  "error": {
    "root_cause": [
      {
        "type": "security_exception",
        "reason": "missing authentication token for REST request [/.kibana/config/6.0.0-alpha1]",
        "header": {
          "WWW-Authenticate": "Basic realm=\"security\" charset=\"UTF-8\""
        }
      }
    ],
    "type": "security_exception",
    "reason": "missing authentication token for REST request [/.kibana/config/6.0.0-alpha1]",
    "header": {
      "WWW-Authenticate": "Basic realm=\"security\" charset=\"UTF-8\""
    }
  },
  "status": 401
}

This causes multiple auth dialogs to show up because Elasticsearch's realm might not match the default realm that Kibana uses, "Authorization Required", and users have to login once per realm.

[spalger]

Fixed by #9214