Don't start transaction during floating IP delete

Don't start transaction during floating IP delete

The previous floating IP code was starting a transaction to
delete the floating IP object from the database and then
calling the core plugin's delete_port method which could make
many yielding calls, risking deadlocks.

This patch updates the relationship between floating IP and the
ports table to allow the port deletion to automatically clear
the floating IP object via a cascade.

Closes-Bug: #1426543
Closes-Bug: #1426482
Change-Id: I4b1cdc198c18d5c673a35aaf9c7e960f97f1e20b