Bug 1590369

Description of problem:
Having the error log file in /var/log means that when missing, daemon (running as mysql:mysql) cannot re-create it. It makes troubles when rotating logs, or when an admin removes it manually. In that case, admin is responsible for re-creating the empty log file now, with proper owner and SELinux context.

I'd like to propose moving the error log to a separate directory properly owned by mysql:mysql user, so daemon can re-created when missing. It's done similarly in mariadb already.

That obviously changes default behaviour, so we should only do it in F29 and further, and document it in Release Notes.. The expected changes are the following:

* For users who have done some changes in /etc/my.cnf.d/community-mysql-server.cnf, the config will stay to use /var/log/mysqld.log
* For users who have not done any changes in /etc/my.cnf.d/community-mysql-server.cnf, or install MySQL on a fresh machine, they will use /var/log/mysql/mysqld.log

So, the impact is not that the service would not start, it's just that some monitoring tools would need to re-configure.