[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/xpack/elasticsearch-xpack.yml

@@ -10,6 +10,7 @@
 #Security configuration
 - name: include security/elasticsearch-security.yml
   include: security/elasticsearch-security.yml
+  when: es_enable_xpack
 
 #Add any feature specific configuration here
 - name: Set Plugin Directory Permissions
@@ -20,4 +21,4 @@
 - name: Set elasticsearch.keystore Permissions
   become: yes
   file: state=file path={{ conf_dir }}/elasticsearch.keystore owner={{ es_user }} group={{ es_group }}
-  when: es_enable_xpack and "security" in es_xpack_features and (es_version is version_compare('6.0.0', '>'))
+  when: es_enable_xpack