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

defaults/main.yml

@@ -1,7 +1,7 @@
 ---
 es_version: "7.7.1"
 es_use_snapshot_release: false
-es_enable_xpack: true
+oss_version: false
 es_package_name: "elasticsearch"
 es_version_lock: false
 es_use_repository: true
@@ -33,7 +33,6 @@ es_action_auto_create_index: true
 es_max_open_files: 65536
 es_max_map_count: 262144
 es_allow_downgrades: false
-es_xpack_features: []
 es_xpack_trial: false
 #These are used for internal operations performed by ansible.
 #They do not affect the current configuration