cos-kernel: Reimplement unlock using cos-set-dev-mode

The existing cos-set-dev-mode script from
project-lakitu/dev-util/cos-dev-tools is much simpler to use than the
pair of scripts that was previously used.

We also simplify the gcloud compute ssh wrapper by using a general retry
pattern, and we move the unlock library functions to pkg/kernel to
more cleanly separate them from the command logic.

BUG=b/527929696
TEST=presubmit, `./cos_kernel unlock kpberry-cos-kernel-test`

Change-Id: I6dfea0436ef0782272152da44151133965c0eab8
Reviewed-on: https://cos-review.googlesource.com/c/cos/tools/+/180646
Reviewed-by: Derek Taylor <ddtaylor@google.com>
Cloud-Build: 228075978874@cloudbuild.gserviceaccount.com <228075978874@cloudbuild.gserviceaccount.com>
Tested-by: Kevin Berry <kpberry@google.com>