run: Add gcp backend

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>

test/test.yml

@@ -30,3 +30,4 @@ outputs:
   - format: kernel+initrd
   - format: iso-bios
   - format: iso-efi
+  - format: gce-img