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.
I am also somewhat undecided. But currently I lean more towards using the same approach as in l3.rb. (Without the ```nova_instance``` attribute). Even if you call that the ugly approch. (Actually I think the ugliness of both approaches is very similar :) )