# defaults/main.yml
...
mysql_server_id: "1"
mysql_max_binlog_size: "100M"
mysql_binlog_format: "ROW"
mysql_expire_logs_days: "10"
mysql_replication_role: master
mysql_replication_master: '127.0.0.1'
# Same keys as `mysql_users` above.
mysql_replication_user:
  - name: replication
    host: "%"
    password: replication
    priv: "*.*:REPLICATION SLAVE,REPLICATION CLIENT"

# tasks/replication.yml
- name: Ensure replication user exists on master.
  mysql_user:
    name: "{{ mysql_replication_user.name }}"
    host: "{{ mysql_replication_user.host | default('%') }}"
    password: "{{ mysql_replication_user.password }}"
    priv: "{{ mysql_replication_user.priv | default('*.*:REPLICATION SLAVE,REPLICATION CLIENT') }}"
    state: present
  when: >
    (mysql_replication_role == 'master')
    and mysql_replication_user
    and (mysql_replication_master != '')
...