Based on #1503
This image wrapped any image with `gsutil` in the path (typically,
`google/cloud-sdk`), and invoked it with our own custom Go wrapper.
That's unnecessary, since `gsutil` can be invoked directly as necessary,
using any image where `gsutil` is in the PATH (`google/cloud-sdk` is the
default).
This change should be entirely transparent to end users.
Operators can override the image being used if they so desire, to point
to any image that contains `gsutil`.
d105b0bec0