Fix FloatingIP.save() passing FixedIP object to sqlalchemy

This prevents the FloatingIP.save() method from passing the
calculated FixedIP object to the sqlalchemy floating_ip_update()
function, which would expect it to be an SA object. It also
aborts any attempt to save the object with a modified fixed_ip_id
linkage, as associate/disassociate should be used for that.

This also fixes a bug where FloatingIP expects the result of
floating_ip_update() to be a FloatingIp SA object.

Change-Id: I065caedf4d81c8583a3b390934a1d403cf2e87bd
Closes-bug: #1334164
Merge "Fix FloatingIP.save() passing FixedIP object to sqlalchemy"