dashboard/coordinator: use VMs to run non-Linux builders

Currently this is running an OpenBSD amd64 builder, but other operating
systems are now easy.

Updates golang/go#8642
Updates golang/go#9492

Change-Id: I4e1026c49ee4d253dfcf78c7606767294784f02e
Reviewed-on: https://go-review.googlesource.com/2264
Reviewed-by: Andrew Gerrand <[email protected]>

Author: bradfitz