Gcutil version bump in May 15 images

We pushed out one last set of Debian (and CentOS) images in connection with Google I/O, where a new gcutil was released. This is the corresponding gcutil version bump for build-debian-cloud.

The build also pulled in gsutil 3.29 and updated google-provided debs without needing any change to build-debian-cloud - those changes just fix things that were previously broken, like 'gsutil update' and running startup scripts stored in Google Cloud Storage, to name a couple, plus improvements to startup script logging. ("Startup script" here is the Google Compute Engine notion, not system sysvinit scripts.)

I think the pace of our PRs will slow to a more normal pace now. :)