roles/mariadb: Use Unix socket for MariaDB tasks

Otherwise Ansible fails due to PyMySQL using a TCP connection.

See: https://github.com/ansible/ansible/issues/47736

roles/mariadb/defaults/main.yml

@@ -19,6 +19,11 @@ innodb_buffer_pool_size: 256M
 # See: https://mariadb.com/kb/en/mariadb/xtradbinnodb-server-system-variables
 innodb_buffer_pool_instances: 1
 
+# Ansible 2.7.x with PyMySQL seems to default to TCP connection so we should
+# force it to use a Unix socket.
+# See: https://github.com/ansible/ansible/issues/47736
+mariadb_login_unix_socket: /var/run/mysqld/mysqld.sock
+
 # default is 100 but the max I've seen used is 5, so let's reduce it
 max_connections: 33