Add a longer ping timeout to ec2 elasticsearch discovery and limit to the our internal security group

[alexlovelltroy]

elasticsearch cloud-aws plugin supports a few options that would make clustering more reliable. We should turn them on by default.

https://github.com/elastic/elasticsearch-cloud-aws#ec2-discovery

I think limiting clustering to our internal security group should be sufficient, but we might want to think about increasing the ping timeout as well.