[check_disk_space] 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 disables the fact gathering as this validation doesn't
use them at all.

Change-Id: I6e4b95faadf2e300030d476c75e6283237b43719
Signed-off-by: Gael Chamoulaud (Strider) <gchamoul@redhat.com>

doc/source/roles/role-check_disk_space.rst

@@ -2,5 +2,50 @@
 check_disk_space
 ================
 
+--------------
+About The Role
+--------------
+
+An Ansible role to check that the partitioning schema on the host(s) has enough
+free space.
+
+Requirements
+============
+
+No requirements.
+
+Dependencies
+============
+
+No dependencies.
+
+Example Playbook
+================
+
+.. code-block:: yaml
+
+   - hosts: localhost
+     gather_facts: false
+     vars:
+       volumes:
+         - { mount: /home, min_size: 25 }
+         - { mount: /, min_size: 50 }
+     roles:
+       - check_disk_space
+
+License
+=======
+
+Apache
+
+Author Information
+==================
+
+**Red Hat TripleO DFG:DF Squad:VF**
+
+----------------
+Full Description
+----------------
+
 .. ansibleautoplugin::
    :role: validations_common/roles/check_disk_space

validations_common/playbooks/check-disk-space.yaml

@@ -1,5 +1,6 @@
 ---
 - hosts: localhost
+  gather_facts: false
   vars:
     metadata:
       name: Verify server fits the disk space requirements

validations_common/roles/check_disk_space/defaults/main.yml

@@ -1,4 +1,8 @@
 ---
+# Volumes partitioning schema
+# A list of dictionary.ies containing:
+# - mount point(s)
+# - min_size (Minimum free space required in GB)
 volumes:
   - {mount: /var/lib/docker, min_size: 10}
   - {mount: /var/lib/config-data, min_size: 3}