xpack features refactoring (#653)

- 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

test/integration/xpack-upgrade.yml

@@ -23,9 +23,6 @@
     es_xpack_license: "{{ lookup('file', '/tmp/license.json')  }}"
     es_plugins:
       - plugin: ingest-attachment
-    es_xpack_features:
-      - security
-      - alerting
     es_api_basic_auth_username: elastic
     es_api_basic_auth_password: changeme
     es_api_sleep: 5
@@ -138,9 +135,6 @@
     es_xpack_license: "{{ lookup('file', '/tmp/license.json')  }}"
     es_plugins:
       - plugin: ingest-attachment
-    es_xpack_features:
-      - security
-      - alerting
     es_api_basic_auth_username: elastic
     es_api_basic_auth_password: elasticChanged
     es_api_sleep: 5