[xpack] use elasticsearch default xpack features (#560)

- Stop forcing es_xpack_features variable in order to let elasticsearch install default features described in http://localhost:9200/_xpack
- Change xpack test scope to be able to test default xpack install
  - xpack scenario will test xpack install with default features
  - xpack upgrade scenario will fully test security feature
  - oss-to-xpack-upgrade will test installing only other specific features
- Cleanup some duplicate serverspec tests
- Remove `system_key`feature (deprecated in 5.6 and removed in 6.0 - [Breaking Changes 6.0.0](https://www.elastic.co/guide/en/elasticsearch/reference/6.0/breaking-6.0.0-xes.html))
- Cleanup some ansible code (especially in `when` conditions)

test/integration/xpack.yml

@@ -14,7 +14,3 @@
     es_xpack_custom_url: "https://artifacts.elastic.co/downloads/packs/x-pack/x-pack-{{ es_version }}.zip"
     es_heap_size: 2g
     es_enable_xpack: true
-    es_xpack_features:
-      - monitoring
-      - graph
-      - ml