DOC - Adopting the common role documentation pattern

As discussed during the Newton design summit, we
agreed to adopt the role documentation pattern laid
out in the os_keystone role

Change-Id: I6a01800a1734872495a60a19de64d3c835574db6

README.rst

@@ -1,18 +1,35 @@
 OpenStack-Ansible Galera Server
 ###############################
-:tags: openstack, galera, server, cloud, ansible
-:category: \*nix
 
 Ansible role to install and configure a Galera cluster powered by MariaDB
 
+Default Variables
+=================
+
+.. literalinclude:: ../../defaults/main.yml
+   :language: yaml
+   :start-after: under the License.
+
+Required Variables
+==================
+
+To use this role, define the following variables:
+
+.. code-block:: yaml
+
+    galera_root_password: secrete
+
+
+Example Playbook
+================
+
 .. code-block:: yaml
 
     - name: Install galera server
       hosts: galera_all
       user: root
       roles:
-        - { role: "galera_server", tags: [ "galera-server" ] }
+        - { role: "galera_server" }
       vars:
-        container_address: "{{ ansible_ssh_host }}"
         galera_root_password: secrete
-        galera_root_user: root
+