Support defining `log_error` in mysql::server

On RedHat 7 the default provider has been set to `mariadb`. However, when one uses MySQL instead, the error log file is still enforced to MariaDB's default (/var/log/mariadb/mariadb.log) which parent folder is obviously not created by MySQL packages, and Puppet fails.

This commit allows setting the path to the error log file to avoid the failure.