util-mariadb: default to binlog MIXED

Newer ansible (2.10) mysql module doesn't like repeated options (with different values?)

fatal: [assenza]: FAILED! => {"ansible_facts": {"discovered_interpreter_python": "/usr/bin/python3"}, "changed": false, "msg": "unable to connect to database, check login_user and login_password are correct or /etc/mysql/conf.d/mynoblogs.cnf has the credentials. Exception message: While reading from '/etc/mysql/conf.d/mynoblogs.cnf' [line 42]: option 'binlog-format' in section 'mariadb' already exists"}