WIP: stack/erp5: Fix mariadb export and import
The exporter.exclude file and restore-from-backup script used when exporting and importing ERP5 as part of Webrunner resiliency were assuming that to re-initialize mariadb it was enough to delete the contents of srv/mariadb but not the folder itself, while the mariadb service assumes that if srv/mariadb exists it is in a valid state and only reinitialises it if it does not exist, meaning it will crash if it exists but is empty.