FloatingIP updateable port_id, fixed_ip_address

This change implements handle_update for FloatingIP.

FloatingIPAssociation already has handle_update implemented, so
this change is just fixing a gap.

Updateable FloatingIP will be vital if ports end up getting replaced
on every update as per I558db6bac196f49e5c488a577f0580c934b06747

Change-Id: I0a05b8cf916267338e4bcb6a3e12bc2cdcdc3d48
Closes-Bug: #1369748
Related-Bug: #1301486
Merge "FloatingIP updateable port_id, fixed_ip_address"