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
Merge "Add FixedIP.floating_ips dynamic property"