dashboard/builder: fallback to finding build keys from GCE metadata
dashboard/builder: fallback to finding build keys from GCE metadata

LGTM=cmang
R=cmang
CC=adg, golang-codereviews
https://golang.org/cl/133610043

runcom/tools,glasser/golang-tools,nicot/tools,maddyonline/tools,runcom/tools,josharian/tools,argandas/tools,springning/tools,golang/tools,nicot/tools,jdhenke/tools,glasser/golang-tools,magiccao/tools,magiccao/tools,cmars/tools,matryer/tools,cmars/tools,glasser/golang-tools,4ad/tools,golang/tools,syreclabs/go-tools,syreclabs/go-tools,nicot/tools,jdhenke/tools,4ad/tools,sshetty10/tools,sshetty10/tools,dmitris/tools,syreclabs/go-tools,argandas/tools,argandas/tools,cmars/tools,cw2018/tools,matryer/tools,maddyonline/tools,syreclabs/go-tools,glasser/golang-tools,nicot/tools,jdhenke/tools,dmitris/tools,syreclabs/go-tools,golang/tools,josharian/tools,magiccao/tools,josharian/tools,magiccao/tools,springning/tools,josharian/tools,dmitris/tools,sshetty10/tools,cw2018/tools,sshetty10/tools,cmars/tools,golang/tools,syreclabs/go-tools,runcom/tools,maddyonline/tools,matryer/tools,dmitris/tools,argandas/tools,matryer/tools,4ad/tools,runcom/tools,cw2018/tools,cw2018/tools,springning/tools,springning/tools,jdhenke/tools,golang/tools,maddyonline/tools