Add VHD and GCE output formats

- VHD is uncompressed VHD. Currently hard coded at 1GB, which may need to change. Use `format: vhd`
- GCE is the GCE compressed tarred raw image. Use `format: gce-img` - reserving `gce` for actually
  uploading the image.

Signed-off-by: Justin Cormack <justin.cormack@docker.com>

base/guestfs/Dockerfile

@@ -1,4 +1,4 @@
-FROM debian:jessie
+FROM debian:stretch
 
 ENV DEBIAN_FRONTEND=noninteractive
 RUN apt-get update && apt-get -yq upgrade && apt-get install -yq libguestfs-tools syslinux linux-image-amd64 vim