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/oss-to-xpack-upgrade.yml

@@ -7,7 +7,7 @@
     - elasticsearch
   vars:
     es_version: "{{ '7.0.0' if es_major_version == '7.x' else '6.7.1' }}" # This is set to an older version than the current default to force an upgrade
-    es_enable_xpack: false
+    oss_version: true
     es_heap_size: "1g"
 
 - name: Standard test for single node setup. Tests idempotence.
@@ -17,9 +17,5 @@
   roles:
     - elasticsearch
   vars:
-    es_enable_xpack: true
+    oss_version: false
     es_heap_size: "1g"
-    es_xpack_features:
-      - monitoring
-      - graph
-      - ml