In this changeset, I added the "goal" ObjectField which can
either be loaded by setting the new "eager" parameter as True or
not loaded (as before) by setting it to False.
The advantage of introducing this eager parameter is that this way,
we can reduce to a minimum the overhead of DB queries whenever the
related goal is not actually needed.
Partially-Implements: blueprint watcher-versioned-objects
Change-Id: I103c9ed161d2cedf7b43c55f9e095ef66bf44dea
f54aca70cc