[check_selinux_mode] Add role documentation in sphinx

This patch adds the documentation directly in the sphinx role
documentation index. All variables have been commented with a proper
description to be included automatically in the sphinx build.

This patch also fixes the facts gathering by being sure we get only the
needed facts.

Change-Id: Ifbe7bd73559b05f6655bfd396029c22345b75f4c
Signed-off-by: Gael Chamoulaud (Strider) <gchamoul@redhat.com>

doc/source/roles/role-check_selinux_mode.rst

@@ -2,5 +2,45 @@
 check_selinux_mode
 ==================
 
+--------------
+About The Role
+--------------
+
+An Ansible role to check SELinux status on the host(s).
+
+Requirements
+============
+
+No requirements.
+
+Dependencies
+============
+
+No dependencies.
+
+Example Playbook
+================
+
+.. code-block:: yaml
+
+   - hosts: localhost
+     gather_facts: false
+     roles:
+       - check_selinux_mode
+
+License
+=======
+
+Apache
+
+Author Information
+==================
+
+**Red Hat TripleO DFG:Security**
+
+----------------
+Full Description
+----------------
+
 .. ansibleautoplugin::
    :role: validations_common/roles/check_selinux_mode

validations_common/roles/check_selinux_mode/tasks/main.yml

@@ -1,4 +1,13 @@
 ---
+- name: Ensure we get needed facts
+  setup:
+    gather_subset:
+      - '!all'
+      - '!any'
+      - '!min'
+      - distribution
+      - os_family
+
 - name: Get current SELinux mode
   command: getenforce
   become: true