[dns] 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
need them at all.

Change-Id: Ica37eb5a2b1edf883d701f523a12eb47264fed7c
Signed-off-by: Gael Chamoulaud (Strider) <gchamoul@redhat.com>

doc/source/roles/role-dns.rst

@@ -2,5 +2,47 @@
 dns
 ===
 
+--------------
+About The Role
+--------------
+
+An Ansible role to verify that the DNS resolution works.
+
+Requirements
+============
+
+No requirements.
+
+Dependencies
+============
+
+No dependencies.
+
+Example Playbook
+================
+
+.. code-block:: yaml
+
+   - hosts: localhost
+     gather_facts: false
+     vars:
+       server_to_lookup: example.com
+     roles:
+       - dns
+
+License
+=======
+
+Apache
+
+Author Information
+==================
+
+**Red Hat TripleO DFG:DF Squad:VF**
+
+----------------
+Full Description
+----------------
+
 .. ansibleautoplugin::
    :role: validations_common/roles/dns

validations_common/playbooks/dns.yaml

@@ -1,5 +1,6 @@
 ---
 - hosts: localhost
+  gather_facts: false
   vars:
     metadata:
       name: Verify DNS

validations_common/roles/dns/defaults/main.yml

@@ -1,2 +1,3 @@
 ---
+# DNS Server to lookup
 server_to_lookup: example.com