Completely object-ify the floating_ips_bulk V2 extension

This converts the floating_ips_bulk APIv2 extension from using
direct DB access to using the FloatingIP object.

Related to blueprint compute-manager-objects-juno

Change-Id: I5fa7637b5eb06a9be51d7638701d98e3e033d68d
Merge "Completely object-ify the floating_ips_bulk V2 extension"