upup: make NodeConfig be JSON-clean
upup: make NodeConfig be JSON-clean

We use JSON-merging in our option parsing, so we need it to only
populate values that are actually set.

WillemMali/kops,mad01/kops,ikropotov/kops,robinpercy/kops,WillemMali/kops,sp-borja-juncosa/kops,aledbf/kops,sethpollack/kops,kubernetes/kops,DualSpark/kops,leandrocr/kops,reactiveops/kops,pdh/kops,justinsb/kops,BradErz/kops,sp-borja-juncosa/kops,faraazkhan/kops,luomiao/kops,luomiao/kops,chrislovecnm/kops,tsupertramp/kops,gambol99/kops,blakebarnett/kops,chrislovecnm/kops,MadDogTechnology/kops,tsupertramp/kops,BradErz/kops,andrewsykim/kops,jnewland/kops,gambol99/kops,mad01/kops,austinmoore-/kops,DualSpark/kops,dacohen/kops,kubernetes/kops,andrewsykim/kops,austinmoore-/kops,luomiao/kops,blakebarnett/kops,ikropotov/kops,DualSpark/kops,mad01/kops,WillemMali/kops,robinpercy/kops,zytek/kops,robinpercy/kops,zytek/kops,reactiveops/kops,ikropotov/kops,faraazkhan/kops,austinmoore-/kops,sethpollack/kops,justinsb/kops,sp-borja-juncosa/kops,tsupertramp/kops,gambol99/kops,faraazkhan/kops,pdh/kops,reactiveops/kops,DualSpark/kops,zytek/kops,pdh/kops,chrislovecnm/kops,faraazkhan/kops,robinpercy/kops,gambol99/kops,MadDogTechnology/kops,austinmoore-/kops,andrewsykim/kops,jnewland/kops,kubernetes/kops,luomiao/kops,tsupertramp/kops,leandrocr/kops,WillemMali/kops,pdh/kops,mad01/kops,zytek/kops,dacohen/kops,kris-nova/kops,reactiveops/kops,jnewland/kops,sethpollack/kops,kris-nova/kops,leandrocr/kops,aledbf/kops,andrewsykim/kops,MadDogTechnology/kops,blakebarnett/kops,MadDogTechnology/kops,blakebarnett/kops,aledbf/kops,sethpollack/kops,justinsb/kops,sp-borja-juncosa/kops,BradErz/kops,dacohen/kops,kris-nova/kops,BradErz/kops,kris-nova/kops,chrislovecnm/kops,aledbf/kops,ikropotov/kops,dacohen/kops,leandrocr/kops