Refactor configuring of floating ips on a router.

This approach to configuring floating ips is stateless and idempotent.
This allows it to handle corner cases, such as reusing a floating ip
address with a different floating ip id in a way that is easier to
understand.

The concept is to wipe the floating ips clean and rebuild them each
time with the following optimizations.  To avoid bad performance in
manipulating iptables, it is called in the context of a call to
defer_apply_on.  To avoid a disruption in network flow a set
difference is use to determine the set of addresses that no longer
belong on the inteface rather than removing them all blindly.

Change-Id: I0cfb58d487b1925e0a0db2a701c5ea3c56a0b2b5
Fixes: Bug #1209011
Merge "Refactor configuring of floating ips on a router."