Apply Floating NAT only on the external interface

Specify the external interface in the Floating IP
iptables NAT rules. This fixes issue when using
Floating IP with extra routes.

WIP:
  - it has to be more tested
  - unit has to be provided

Change-Id: Idda448c9eaeb43e83d6b0f4b5da0239ccad3cdbd
Closes-bug: #1375994