Create user _galera in galera-arbitrator-4 instead of galera-4
The galera-4 binary package is a plugin for mariadb: this library already runs in the context of the mariadbd daemon: typically as mysql user and doesn't require any specific user for itself.
The standalone garbd arbitrator daemon usually runs on a system where mariadb-server and thus galera-4 is not installed. Because of this, systemd fails to start garbd because it doesn't find the _galera user now added in garbd.service's configuration.
Simply move the postinst script creating _galera from galera-4 to galera-arbitrator-4 to fix this.
Signed-off-by: Adel Belhouane bugs.a.b@free.fr