gce: Enable GCE from the repo system

With this patch, the following cmd will create a GCE instance directly from
the remote GCE url, e.g., gs://osv/v0.09/osv-iperf.gce.tar.gz.

$ capstan run -p gce cloudius/osv-iperf

The url is obtained from a file in the remote repo.

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