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
c6cd605781