Updated `gcloud` image-related flags.
Updated `gcloud` image-related flags.

The flag `--image` which uses image aliases has been deprecated and replaced
with `--image-project` and `--image-family`. For more info, see:

* https://cloud.google.com/compute/docs/images
* https://cloud.google.com/sdk/gcloud/reference/compute/instances/create

This addresses isse #10.

mbrukman/cloud-launcher,mbrukman/cloud-launcher,mbrukman/cloud-launcher,mbrukman/cloud-launcher