[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/elasticsearch-parameters.yml

@@ -20,7 +20,10 @@
 #Check if working with security we have an es_api_basic_auth_username and es_api_basic_auth_username - otherwise any http calls wont work
 - name: fail when api credentials are not declared when using security
   fail: msg="Enabling security requires an es_api_basic_auth_username and es_api_basic_auth_password to be provided to allow cluster operations"
-  when: es_enable_xpack and ("security" in es_xpack_features) and es_api_basic_auth_username is not defined and es_api_basic_auth_password is not defined
+  when:
+    - es_enable_xpack and "security" in es_xpack_features
+    - es_api_basic_auth_username is not defined
+    - es_api_basic_auth_password is not defined
 
 - name: set fact file_reserved_users
   set_fact: file_reserved_users={{ es_users.file.keys() | intersect (reserved_xpack_users) }}