[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/serverspec/default_spec.rb

@@ -1,9 +1,7 @@
-require 'xpack_spec'
 require 'shared_spec'
 require 'json'
 vars = JSON.parse(File.read('/tmp/vars.json'))
 
 describe 'Xpack upgrade Tests' do
   include_examples 'shared::init', vars
-  include_examples 'xpack::init', vars
 end