image: Add GCE image support

GCE uses tar.gz image format.

Signed-off-by: Asias He <asias@cloudius-systems.com>
gce: Initial Google Compute Engine support

$ capstan run  -p gce osv.tar.gz
Created Instance: osv-capstan-1396312152
Public        IP: 162.222.016.100
Internal      IP: 10.240.100.100
Machine     Type: n1-standard-1
Zone            : us-central1-a

This will upload the OSv image tarball and create an instance.

To ssh into instance:
$ gcutil ssh osv-capstan-1396312152

To get serial output:
$ gcutil getserialportoutput osv-capstan-1396312152

TODO:
- Add option to specify Google Cloud Storage bucket to use, instance
  type, instance zone, etc.
- Support creating instance fom remote a Google Cloud Storage url, e.g.,
  $ capstan run -p gce gs://osvimg/osv.tar.gz
- Support creating instance fom remote a Google Compute Engine image
  name, e.g.,
  $ capstan run -p gce my_image_name

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