dashboard/coordinator: use new dashboard JSON interface to find work
dashboard/coordinator: use new dashboard JSON interface to find work

This uses the new JSON interface to the build dashboard (from
golang.org/cl/2290) to find all work, and re-enables the OpenBSD
builder[*], and can do multiple things at a time. Andrew and I just
watched it fire up 8 OpenBSD VMs at once to catch up.

[*] The OpenBSD builder was disabled because it would only report
results for the main repo, not subrepos, and the old
build.golang.org/todo interface didn't understand that was
possible. Now the steps are considered separate.

Update golang/go#8642 (OpenBSD)
Update golang/go#9492 (builds in VMs)

Change-Id: Ic6c2f73ee3da218dd54ef1a33f3afc97046ea3cc
Reviewed-on: https://go-review.googlesource.com/2282
Reviewed-by: Andrew Gerrand <395a7d33bec8475c9b83b7d440f141bcbd994aa5@golang.org>

golang/build,evandbrown/build,proppy/build,golang/build,proppy/build,proppy/build,golang/build,golang/build,proppy/build,evandbrown/build,evandbrown/build