Fix getting nwinfo for Instance obj

This fixes the get_nw_info_for_instance() method in nova/compute/utils.

If an instance object has no 'info_cache' (means we lack an entry in the
instance_info_caches table for the instance), don't try to access the
'network_info' attribute.  Just return a network model with empty
networking information like the dict version of an instance would do.

Change-Id: Ib1f49f10cefeefc55a87fea69b6fbc772c978dca
Closes-bug: 1243291
Merge "Fix getting nwinfo for Instance obj"