[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/oss-to-xpack-upgrade.yml

@@ -20,8 +20,8 @@
   vars:
     es_instance_name: "node1"
     es_enable_xpack: true
-    es_api_basic_auth_username: elastic
-    es_api_basic_auth_password: changeme
     es_heap_size: "1g"
     es_xpack_features:
-      - security
+      - monitoring
+      - graph
+      - ml