Disable SSH password auth on GCE

This change was unintentional but occurred as part of GCE's transition
from build-debian-cloud to bootstrap-vz.

Might be replaced later with a similar change that applies to all
bootstrap-vz providers, based on the opinion of the debian-cloud list.

Change-Id: I72a694c49f32df06252d9cc01b1d5c7cfc015347
Removed a check to verify file type

The file command is not installed by bootstrap-vz by default.

Change-Id: I59d5f04145b7db517c28bfec1d4d758be468b398