Fix attr when ready rebind 1422

fix for #1422 - @aledsage @sjcorbett does this make sense?

it is just the last commit https://github.com/ahgittin/brooklyn/commit/ccfdfa430f2a69367c06f52b6e2768f49969bfd6 being submitted here, but it has some merge conflicts if we unpick #1452 so i've built on that.