Fixed two separate bugs in --gce-kernel addition

These bugs had prevented --gce-project from adding a Google Compute
Engine image successfully since the commit to add support for
--gce-kernel. Now it ought to work. :) These bugs did not interfere with
the rest of the process, including the upload step to Google Cloud
Storage, or with any of the process for other clouds.
Preinstall acpi-support-base for gce target

Google Compute Engine sends an ACPI power button press signal to power
down VMs cleanly. Debian supports this, but only with acpi-support-base
installed.  This addition reduces Debian VM shutdown time from ~2
minutes to ~20 seconds.
Support for > 1 apt mirror, use in Compute Engine

Google Compute Engine images will now fall back to http.debian.net if
apt finds a problem with the Google-cloud-internal mirror.
Clean up Google-specific repository during build

The repository is not yet GPG-signed and from a hostname which end users
might not expect to host an official Google package repository,
especially after apt prompts them with an "unauthenticated packages"
warning. To ensure user confidence without training them to ignore
warnings, remove the extra apt source before finalizing the image.
More clarity + flexibility for --gcs-dest

While --gcs-dest did already work, it previously added a slash to the end of
the specified URL prefix, even if the user already ended the parameter with one.
Refrain from doubling a single terminal slash. Also clarify the nature
of the parameter from URL to URL prefix; it is an URL and a prefix, but the
final URL will have a slash added if necessary followed by the image tarball
name.