Make nova-network use FixedIP for get_by_address() queries

This makes nova-network use the FixedIP object for get_by_address()
queries.

This also fixes a bug in test_manager that incorrectly had
fixed_ip.network_id as a UUID string in the fake IP objects.

Related to blueprint nova-network-objects

Change-Id: I732e4d7df76e5ae7bd010dfa6b0d8f8dd0c69454
Merge "Make nova-network use FixedIP for get_by_address() queries"