coordinator: add support for different build dashboards
coordinator: add support for different build dashboards

LGTM=bradfitz
R=bradfitz
CC=adg, golang-codereviews
https://golang.org/cl/135520043

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