Move backup-related variable defaults from the database roles into the
borg_client role. Also check group membership to guard installation of
database backup helper scripts.

Due to the "systemctl is-active foo && backup-foo || true" shorthand,
errors during database dumping were being ignored. Change the MariaDB
section to also be wrapped in a proper if statement. Finally, get rid
of "|| true" silencing statements + enable errexit in helper scripts.