- rename es_enable_xpack variable to oss_version
this variable name should be more explicit and consistent with the xpack changes made in 6.3
es_enable_xpack can still be used but is deprecated and will be removed in a later version.
- remove es_xpack-features variables
this variable was not working anymore since 6.3 [0]
[0]: https://github.com/elastic/ansible-elasticsearch/issues/608