scripts: Zero download GCE support

Generate a url file for GCE image type.

For example,

$ ls build/capstan/cloudius/osv-iperf
1) index.yaml
2) osv-iperf.gce.gz
3) osv-iperf.qemu.gz
4) osv-iperf.vbox.gz
5) osv-iperf.vmw.gz
6) osv-iperf.gce.tar.gz

Files 1) to 5) will be copied to s3 repo. File 6) will be uploaded to
Google Compute Engine.

$ zcat build/capstan/cloudius/osv-iperf/osv-iperf.gce.gz
gs://osv/v0.09-10-g140ac67/osv-iperf.gce.tar.gz

With this, the following cmd will create a GCE instance directly from
the gs://... url.

$ capstan run -p gce cloudius/osv-iperf

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