When using local.conf in multinode envs not everything is going to be
defined in all places. Eventually we probably want to make it so we
have a host role for these sections or something. But for now warn
instead of die when we can't find a config var.
Change-Id: I6959099373f035fbfe9e540a44e4c52b8e7c95c0
Closes-Bug: #2000824