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.
Hrm. In the l3.rb recipe, we simply do this:
```
#TODO: nova should depend on neutron, but neutron also depends on nova
# so we have to do something like this
novas = search(:node, "roles:nova-multi-controller") || []
if novas.length > 0
  nova = novas[0]
  nova = node if nova.name == node.name
else
  nova = node
end
```

Using `get_instance` is obviously better, but I'm worried about using a `nova_instance` attribute for a dependency that we're happy to not depend on (at the orchestration level of crowbar). That is, even if the attribute is set to `"default"`, we ignore it for ordering.

So I'm mixed between using the clean approach you're suggesting but breaking part of the crowbar model, and the ugly approach that we already have.

@aspiers, @rhafer what do you think?