Support DNS load balancing while connecting to elasticsearch

[dernasherbrezon]

Kibana version: 5.5.0

Elasticsearch version: 5.5.0

Server OS version: rhel 64bit

Browser version: chrome

Browser OS version: windows

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

Description of the problem including expected versus actual behavior:
http.Agent doesnt support DNS load balancing. According to nodejs/node#708 node doesn't support multiple IPs out-of-the-box

Steps to reproduce:

1. Configure elasticsearch for high availability using DNS load balancing (i.e. dig elastic.example.com should return at least 2 IPs ( client -> dns -> ip proxy -> elastic)
2. Configure kibana to use elastic.example.com
3. Shutdown one ip proxy.
4. Open kibana -> Dev tools
5. Run "GET /_cat/indices?v" several times
6. Sometimes command will return list of indices, sometimes it will return:
   {
   "statusCode": 502,
   "error": "Bad Gateway",
   "message": "connect ECONNREFUSED ip:80"
   }

curl and browser work fine.

[jbudz]

Related to #214. There's some discussion there if you want to take a look, we usually suggest adding a client node that Kibana connects to, although there's still the single point of failure unfortunately.

Modified the title slightly to avoid ambiguity between this and #214

[tylersmalley]

I don't think anything needs to be done here since #214 is closed. I agree with Jon that the best solution, when possible, is often to have a local ES coordinating node that Kibana connects to.