[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)

defaults/main.yml

@@ -29,7 +29,7 @@ es_max_open_files: 65536
 es_max_threads: "{{ 2048 if ( es_version is version_compare('6.0.0', '<')) else 8192 }}"
 es_max_map_count: 262144
 es_allow_downgrades: false
-es_xpack_features: ["alerting","monitoring","graph","ml","security"]
+es_xpack_features: []
 #These are used for internal operations performed by ansible.
 #They do not affect the current configuration
 es_api_host: "localhost"