gce: Enable gs://bucket/image.tar.gz image type

Support start an instance directly from an Google Storage URL. This
enables zero download and upload to start an instance. E.g.:

$ capstan run -p -i gs://osv/osv.tar.gz osv_instance

Signed-off-by: Asias He <asias@cloudius-systems.com>
gce: Add ssh info

Print ssh login info to hep user login with ssh into OSv.

Signed-off-by: Asias He <asias@cloudius-systems.com>
gce: Store VMConfig to a file

We can use this file to restart a stopped instance.

Signed-off-by: Asias He <asias@cloudius-systems.com>
gce: Add delete support

The instance is deleted and root disk is deleted as well.

Signed-off-by: Asias He <asias@cloudius-systems.com>
gce: Add stop support

The instance is stopped and the root disk of the instance is not
deleted.

Signed-off-by: Asias He <asias@cloudius-systems.com>
gce: Enable restart an stopped instance

To stop an instance:
$ capstan stop $instance_name

And then start an instance:
$ capstan run -p gce $instance_name

Signed-off-by: Asias He <asias@cloudius-systems.com>
gce: Display instance status correctly

Query the instance status on the fly which will give the correct
instance status: Running or Stopped.

Signed-off-by: Asias He <asias@cloudius-systems.com>
cmd/run: Cleanup print out for instance creating

Signed-off-by: Asias He <asias@cloudius-systems.com>
cmd/instance: Align name filed properly

Instance names might be longer than 20, e.g.,
cloudius-osv-memcached-opt. Align it properly.

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