releng-gce: Add ZFS support to GCE image buliding and uploading

Approved by: delphij (re), gjb (re)
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D41527

(cherry picked from commit 1b10bdf0e2fa80a485980b783469687623d3f1c3)
(cherry picked from commit c892a9af7187e94f8dd6f720609e84d79c9e7d1f)