Use new gcutil deb from Google's repository
Update URL of Google deb repository
Prefer newly released kernel on Google cloud
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.
Install ca-certificates. Fixes #111
Adding flags for supporting backports kernel
Adding a preference of backports kernel when requested
Updating backport sources when backport is requested
GCE deb rename: image_bundle->python-gcimagebundle
Add script changing NTP server address.
Fixes after Jimmy Kaplowitz review:

 * use sed with -i option instead of grep to avoid creation of temp file
 * add comment to ntp.conf file explaining configuration change
 * change comments explaining that NTP server is located on host machine
Remove existing lines with server addresses instead of commenting them.
Do not force sed to create backup copy.
Initially set the hostname to localhost to workaround startup error.
Better comment for setting default hostname.
Add note about bootstrap-vz