Use update_serial as an ansible variable

Use update_serial as ansible variable for being able to
override the value with CLI or ansible-playbook command line.

This should result as the following:
at the heat level:
  * if no update_serial in the roles_data is provided, the default
    value is 1
  * if update_serial is provided by the roles_data, the default value
    for the ansible var is the one in the roles_data.
  * if ansible-playbook -e update_serial=<something> , the serial value
    is the one provided by the CLI.

Change-Id: Id4a6326977bebba56c3da2dbf3c4113b6658d433
(cherry picked from commit c6cd605781)

common/deploy-steps.j2

@@ -560,7 +560,7 @@ outputs:
 {%- for role in roles %}
         - hosts: {{role.name}}
           name: Run update
-          serial: {{ role.update_serial | default(1) }}
+          serial: {{ '"{{' }} update_serial | default({{role.update_serial | default(1)}}) {{ '}}"' }}
           gather_facts: no
           vars:
             tripleo_minor_update: true