Upgrade to elastic 2.3

[pmorton]

Hi - I have done some work to update the plugin!

1. Elastic 2.3 Support
2. Use AWS credential provider chains to auto-detect aws credentials
   • Note that this removes the ability to specify credentials in config. They either need to be in and env var, java properties or an instance profile.
3. Add more metrics
4. Normalize metric sending on the node and index level.

I understand that this may diverge from your intention with the original plugin, so please feel free to close the PR if it's not in line with the direction you are taking things.