Revert "Remove unnecessary code."
Revert "Remove unnecessary code."

This reverts commit 382801954ded501829a7a317c037a289f92099b2.

This code is not unnecessary, the first RetrieveNode call is a fail-fast
path for when the node does not exist, but we cannot trust the node
contents until we've acquired a lock to it.

tsuru/docker-cluster,tsuru/docker-cluster