release: tweak Oracle Cloud settings

• use raw image disk type and enable zfs, this yields smaller images for upload after using native qcow2 + zstd compression

Reviewed by: lwhsu, emaste
Differential Revision: https://reviews.freebsd.org/D47055
MFC after: 3 days
Approved by: emaste

(cherry picked from commit c611041660845e8ee8bf3009b32327d1f783a47b)