Remove multi instances support (#566)

* remove multi instances support
The goal is to stop supporting installation of more than one node in the same host. This commit update the Ansible role README documentation and remove the multi instances kitchen test.

* remove systemd and init.d templates
As we no more need to support more than one node on the same host, we no more need to override init files provided by elasticsearch official packages.

* remove file script feature
File scripts have been removed since elasticsearch 6.0 (https://www.elastic.co/guide/en/elasticsearch/reference/6.0/breaking_60_scripting_changes.html#_file_scripts_removed)

* remove custom user and custom group
ES_USER and ES_GROUP settings are no longer supported (https://www.elastic.co/guide/en/elasticsearch/reference/6.0/breaking_60_packaging_changes.html#_configuring_custom_user_and_group_for_package_is_no_longer_allowed)

* add upgrade procedure

* use same task for license activation with and without authentication

tasks/xpack/elasticsearch-xpack.yml

@@ -20,5 +20,5 @@
 #Make sure elasticsearch.keystore has correct Permissions
 - name: Set elasticsearch.keystore Permissions
   become: yes
-  file: state=file path={{ conf_dir }}/elasticsearch.keystore owner={{ es_user }} group={{ es_group }}
+  file: state=file path={{ es_conf_dir }}/elasticsearch.keystore owner={{ es_user }} group={{ es_group }}
   when: es_enable_xpack