Fetch elasticsearch network host interface a bit more dynamically

[patcon]

https://github.com/rackspace-cookbooks/elkstack/blob/master/attributes/elasticsearch.rb#L25

This is currently hardcoded for rackspace, and while a wrapper cookbook can easily fix it, would be nice if it were a bit more dynamic.

Should be able to test for the existence of automatic attr node['cloud_v2']['local_ipv4'] to determine whether to try to use the private network interface eth1, right? (digitalocean doesn't enable this by default)

[martinb3]

Seems like the same issue as #52.

[martinb3]

I'd definitely like to see us improve on this.

[patcon]

Ah thanks. Closing as dup