For legacy router, there are some iptables rules added for external gateway
port. Some of these rules are for shared snat, some are for floating ip.
When user disables shared snat of router gateway, some of the iptables rules
that floating ip needs will not be added to router. This will cause the
reported bug, ping floating ip but reply with fixed ip.
The fix will add the iptables rules that floating ip needs, no matter if
router enables shared snat. A functional test is also added for the issue.
Change-Id: I3cf4dff90f47a720a2e6a92c9ede2bc067ebd6e7
Closes-Bug: #1551530
cea149212e