[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)

tasks/main.yml

@@ -68,7 +68,10 @@
 
 - name: set fact manage_native_realm to true
   set_fact: manage_native_realm=true
-  when: es_start_service and (es_enable_xpack and "security" in es_xpack_features) and ((es_users is defined and es_users.native is defined) or (es_roles is defined and es_roles.native is defined))
+  when:
+   - es_start_service
+   - es_enable_xpack
+   - (es_users is defined and es_users.native is defined) or (es_roles is defined and es_roles.native is defined)
 
 # If playbook runs too fast, Native commands could fail as the Native Realm is not yet up
 - name: Wait 15 seconds for the Native Relm to come up