This commit implements `moby run gcp` which allows for testing of moby
images on the Google Cloud Platform
This backend attaches (via SSH) to the serial console.
It generates instance-only SSH keys and adds the public key to the
image metadata. These are used by the `moby` tool only.
It will also automatically upload a file and creates an image if the prefix
given to `moby run` is a filename
Signed-off-by: Dave Tucker <dt@docker.com>