osds: change default value for `dedicated_devices`

osds: change default value for `dedicated_devices`

This is to keep backward compatibility with stable-2.2 and satisfy the
check "verify dedicated devices have been provided" in
`check_mandatory_vars.yml`. This check is looking for
`dedicated_devices` so we need to default it's value to
`raw_journal_devices` when `raw_multi_journal` is set to `True`.

Fixes: https://bugzilla.redhat.com/show_bug.cgi?id=1536098
Signed-off-by: Guillaume Abrioux <gabrioux@redhat.com>