borg/backup-my.cnf: Quote password

The password for luna contains some special chars that apparently don't
work properly without quotes.
Signed-off-by: Florian Pritz <bluewind@xinu.at>

roles/borg-client/templates/backup-my.cnf.j2

 [client]
 user = root
-password = {{mariadb_users.root}}
+password = "{{mariadb_users.root}}"