Handle GPG key for Compute Engine apt repository

Google's repository is now GPG-signed, so builds for Google Compute Engine have
to handle the key for apt not to fail on the unknown key.
Compute Engine: prepare for Debian kernel support

Google Compute Engine has not yet launched the support required for the Debian
kernel, but these changes are in preparation for when that does occur. Until
then, our images will continue to work with the Google-provided kernels.
Remove spurious use of kpartx

A previous internal iteration of commit
a79d2e490671e8aa465b4215a9e570d3d3f6af69 used kpartx, but the final version
didn't. Remove the logic in the cleanup code which tries to tear down kpartx.
Ironically this would fail on a system without kpartx installed, and it's not
needed as a dependency in the final version.
Merge pull request #102 from google/master

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