Customization: Add an explicit example of disabling Floating IPs

The current documentation on customizing focuses on
registering/unregistering components.  While this is simple and
effective at a high-level, fine-grained customizations like overriding
individual allowed() methods uses a different approach.

This confuses n00bcakes like myself, so add an explicit example of
dropping Floating IPs support to spell it out.

Fixes Bug: 1210419

Change-Id: Ib9b1579b33d48c7b2bd9bf3ad96cd303210a4420
Merge "Customization: Add an explicit example of disabling Floating IPs"