[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-upgrade.yml

@@ -16,7 +16,6 @@
       xpack.security.authc.realms.native1.type: native
     es_config_7x:
       http.port: 9200
-      xpack.security.enabled: True
       xpack.security.authc.realms.file.file1.order: 0
       xpack.security.authc.realms.native.native1.order: 1
     es_config: "{{ es_config_7x if es_major_version == '7.x' else es_config_6x }}"
@@ -33,7 +32,6 @@
       - alerting
     es_api_basic_auth_username: elastic
     es_api_basic_auth_password: changeme
-    es_message_auth_file: system_key
     es_role_mapping:
       power_user:
         - "cn=admins,dc=example,dc=com"
@@ -131,7 +129,6 @@
       xpack.security.authc.realms.native1.type: native
     es_config_7x:
       http.port: 9200
-      xpack.security.enabled: True
       xpack.security.authc.realms.file.file1.order: 0
       xpack.security.authc.realms.native.native1.order: 1
     es_config: "{{ es_config_7x if es_major_version == '7.x' else es_config_6x }}"