xpack features refactoring (#653)

- rename es_enable_xpack variable to oss_version
  this variable name should be more explicit and consistent with the xpack changes made in 6.3
  es_enable_xpack can still be used but is deprecated and will be removed in a later version.
- remove es_xpack-features variables
  this variable was not working anymore since 6.3 [0]

[0]: https://github.com/elastic/ansible-elasticsearch/issues/608

tasks/compatibility-variables.yml

@@ -5,6 +5,14 @@
 # 2. When running multiple plays with the same role the defaults are not re-evaluated. An example of this
 # can be seen in our the https://github.com/elastic/ansible-elasticsearch/blob/master/test/integration/xpack.yml
 # integration test and in the Multi Node server documentation examples https://github.com/elastic/ansible-elasticsearch/blob/master/test/integration/xpack.yml
+- name: Set backward compatibility for deprecated es_enable_xpack variable
+  when: es_enable_xpack is defined and not es_enable_xpack
+  block:
+  - name: Set fact oss_version when using es_enable_xpack
+    set_fact: oss_version=true
+  - name: Warn about deprecated es_enable_xpack variable
+    debug:
+      msg: "WARNING: es_enable_xpack variable is now deprecated. You should use oss_version instead"
 
 - name: Set the defaults here otherwise they can't be overriden in the same play if the role is called twice
   set_fact:
@@ -14,7 +22,7 @@
     es_other_repo_name: "{{ 'oss-' + es_major_version }}"
     es_other_apt_url: "deb {{ es_repo_base }}/packages/{{ 'oss-' + es_major_version }}/apt stable main"
 
-- name: Use the oss repo and package if xpack is not being used
+- name: Use the oss repo and package
   set_fact:
     es_repo_name: "{{ 'oss-' + es_major_version }}"
     es_other_repo_name: "{{ es_major_version }}"
@@ -22,7 +30,7 @@
     es_package_name: "elasticsearch-oss"
     es_other_package_name: "elasticsearch"
   when:
-    - not es_enable_xpack
+    - oss_version
 
 - name: Set the URL scheme to https if SSL/TLS is enabled
   set_fact: