Make nova-network use FixedIP for host and instance queries

This makes nova-network manager use the FixedIP object for the
get_by_network_and_host() and get_by_instance_uuid() queries.

Related to blueprint nova-network-objects

Change-Id: Ifa26ba1db6a9975e21a5a1c4b8742d82ca6d0772
Merge "Make nova-network use FixedIP for host and instance queries"