Add FixedIP.floating_ips dynamic property

This adds a lazy-loading FixedIP.floating_ips property which mimics
the use of the backref between the two objects in the SQLAlchemy
model. This doesn't introduce a new database query because it was
being used as a lazy-load in network manager already.

Related to blueprint nova-network-objects

Change-Id: I2f13f7a0463486e51c2317e02c2e7b378ac9e55d
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 "Add FixedIP.floating_ips dynamic property"
Merge "Make nova-network use FixedIP for get_by_address() queries"