Replication should set a binlog format

This could possible be better served in the base config file, but
for now I'll keep it here.

mysql/defaults/main.yml

@@ -45,6 +45,7 @@ mysql_restart_if_needed: True
 # replication variables:
 mysql_replication: false
 mysql_log_bin: null
+mysql_binlog_format: mixed
 mysql_server_id: null
 mysql_bind_address: null
-mysql_repl_password: ''
+mysql_repl_password: ''

mysql/templates/replication.cnf.j2

@@ -5,3 +5,4 @@
 log_bin = {{ mysql_log_bin }}
 {% endif %}
 server_id = {{ mysql_server_id }}
+binlog_format = {{ mysql_binlog_format }}