Changes from May 9 images

Meant to submit these a few days ago but didn't. All of these commits have more details in the individual commit messages and are live in last Thursday's Google Compute Engine images.

- More clarity + flexibility for --gcs-dest
- Clean up Google-specific repository during build
- Support for > 1 apt mirror, use in Compute Engine
    - we now have ideas on how to add support for our mirror into http.debian.net, and talking to Raphael is planned
    - The --apt-mirrors option is available for Amazon EC2 as well. Default is still http.debian.net there.
- Preinstall acpi-support-base for gce target
- Fixed two separate bugs in --gce-kernel addition

Later this week I'll have another PR to submit, to include newer Google code and maybe another tiny thing or two. The build-debian-cloud diff for that will be much smaller than this one.