OS-246: create valid instance record

We were creating instance database records with
instance['info_cache']['network_info'] == None, then a moment later,
setting the network_info. If you ran a 'nova list' in between these
transactions, nova-api would raise an exception. This patch fixes the
problem by creating the database record with a single transaction.

Change-Id: I75854f17de82b4e6df8f8e7f70673f76c94cf4f6