Ensure .status_code is defined for all NeutronClientExceptions

nova contains plenty of code like:
  except neutron_client_exc.NeutronClientException as e:
      if e.status_code == 409:
          ...

This change declares a fallback of 0 for exceptions without a more
explicit status code.  This was the behaviour prior to Change-Id
I99b9560b3afaf5884fd00353323267da450338fa

Change-Id: Ib736205066fd5cd9738fd3fb3b64f6590803cf39
Merge "Ensure .status_code is defined for all NeutronClientExceptions"