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.
For the record, I've added for now a patch to the package with the other way since it does not require a new attribute -- that allows switching to any of the two solutions easily at a later point in time.