Make nova-network use Network object for get-all-by-host query

This makes nova-network use the NetworkList.get_by_host() instead
of the direct-to-database query.

Related to blueprint nova-network-objects

Change-Id: Id94415ea650bce8009ea2db98f5d175206f7e509
Merge "Make nova-network use Network object for get-all-by-host query"