config_sync_cluster always uses the mysql database

The database used in the configuration synchronization is always the mysql database. This makes the use of the feature very strict and also prevents various features from being used with it due to where it is created. For example, creating triggers on this table is not possible as MariaDB forbids the creation of them on any table inside the mysql database, including user-created ones.

The simple solution to this is to make the database configurable. The proposed parameter, config_sync_db, would still default to mysql but it would at least make it configurable. This would also allow the configuration synchronization user to be created with a reduced set of grants that only relate to the configured database.