Remove /etc/resolv.conf during image creation

The build system's DNS info should not pollute the image. DHCP will
recreate this for the VM environment.

Also renumber tasks/gce/52-* to tasks/gce/25-* so that those steps occur
when DNS is still functional inside the image.
Clean up loopback devices and mounts on all exits

Previously, depending on where a Google Compute Engine image build
failed, the loopback mount could linger, which is cumbersome. This
cleans that up, though it leaves the workspace around during unclean
exits to facilitate debugging.