INCOMPLETE and untested, do not use!
https://mariadb.com/kb/en/securing-communications-in-galera-cluster/
https://severalnines.com/blog/how-and-performance-impact-ssl-encrypted-replication-traffic-galera-cluster-mysql
https://severalnines.com/blog/database-security-fully-ssl-encrypt-mysql-galera-cluster-clustercontrol