Configure floating IPs addresses after NAT rules

Change the behaviour of the L3 agent in order to set the IP addresses
for the floating IPs on the external gateway interface after the
relevant NAT rules have been applied.
This will avoid a transitory period in which the floating IP exists
and is reachable but it not yet wired to the actual target.

Partial-Bug: #1265505

Change-Id: Ib382fde021868bab2185f2fa5bdee86559148ba7
Merge "Configure floating IPs addresses after NAT rules"