fix(peer join) fix wrong join command redirection
1. We use PUT request to do a V2 join. So we should redirect a PUT request rather than a POST.
2. /admin only accept V2Join request. Send out V2Join instead of V1Join.
Merge pull request #717 from xiangli-cmu/fix_join_redirection

fix(peer join) fix wrong join command redirection
docs(production-ready): explain the state of etcd clearly
Merge pull request #718 from bmizerany/master

docs(production-ready): explain the state of etcd clearly