Make floating_ips module use FloatingIP for (de-)allocations

This makes the network.floating_ips module use the FloatingIP
object for allocation and deallocation.

Related to blueprint nova-network-objects

Change-Id: I10d334d13b77be4bc8d491644fad6394798cd76b
Merge "Make floating_ips module use FloatingIP for (de-)allocations"