Fix search for nova instance

When nova-multi-controller and neutron-server are deployed
on two different nodes, the neutron.conf was not properly updated
with the nova settings because it could't find the node. Search
for the instance (optional since normal deployment order currently
is neutron first, then nova) properly so that get_instance()
in the chef recipe can do its job.
yeah, this is a tricky problem indeed. As it stands today nova and neutron have circular dependencies on each other, so we gotta break it somewhere. 

for our environment where we allow only one proposal both variants should be fine I guess. but if you have multiple nova proposals then this PR has a slight advantage.