Fix two issues that were uncovered by GCE's 20140807 image

- My included commit: Once again disable SSH password auth. GCE isn't currently using the SSH task group, and while this might be convenient to change if it can be made to meet GCE's needs, we should restore the intended security settings first.
- My colleague @illfelder's included commit: I had put in a call to the 'file' command to debug my Cloud SDK pull request. This was present on images built by build-debian-cloud but for some reason is not preinstalled bootstrap-vz. Therefore we were able to run bootstrap-vz on a pre-transition image but not on an image built by bootstrap-vz. His removal of my call to 'file' fixes this issue.

We've done internal test builds which confirm that these fixes work. We hope to publish revised public images soon, either those test builds themselves or subsequent iterations depending on any other issues that might be discovered.