scripts/build-capstan-img: Build GCE image

Signed-off-by: Asias He <asias@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
scripts/build-capstan-img: Build VMW image

Signed-off-by: Asias He <asias@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
build-osv-release: Switch to empty image

Currently, "mgmt.httpserver" pulls the entire OpenJDK which is not what
we want for osv-base image which is supposed to be around 10 MB.

Switch to "empty" image configuration until the problem is resolved.

Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>