Two Google Compute Engine fixes: one bugfix, one future prep

For once I'm pushing commits before we put them into an image!

- "Handle GPG key for Compute Engine apt repository": The repository was unsigned as of when we built our August 16 Debian image (and that tarball was reused with a different kernel for the September 26 image), so it wasn't necessary for our published builds so far. Since we started signing it, private builds might have broken without handling the key. This will also enable future public builds to work.
- "Compute Engine: prepare for Debian kernel support": Mostly self-explanatory purpose. :) The images still use a Google-injected kernel until the feature launches.
    - I also submitted a third commit updating this preparatory commit to remove some accidentally included cleanup code relating to kpartx, since despite some internal experiments I didn't end up introducing a kpartx dependency into the script after all.