dashboard/env: add script to create OpenBSD GCE image

This is almost entirely the work of @mdempsky; I just ran
it a bunch and reported bugs or tweaked things until it worked
and booted the buildlet on GCE.

Update #8642

Change-Id: If5d50bcdf90bc6aa648472cc951ed1c53b46eef3
Reviewed-on: https://go-review.googlesource.com/2156
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>