[elasticsearch] Update config for 6.x

This updates the elasticsearch configuration file (and loading
mechanism) for ELK 6.x.

The default location for the configuration for all package
distributions is /etc/elasticsearch[1], so now that is where we
overwrite the elasticsearch.yml.

The path.conf and path.scripts paths are no longer supported and will
raise exceptions if utilized in 6.x.

[1]:
https://www.elastic.co/guide/en/elasticsearch/reference/6.x/settings.html#config-files-location

Implements: blueprint elasticsearch-kibana-version-upgrade
Depends-On: https://review.opendev.org/#/c/647748/
Change-Id: I4f74bfe07d4b7ca18953b11e767cf0bb94dfd67e