Don't snat traffic between fixed IPs behind same router

Don't snat traffic between fixed IPs behind same router

This fixes a bug where an iptables rule to not snat traffic between
fixed IPs is only being added if enable_snat=true. We should add
this rule no matter what the value is for enable_snat.

Without this patch, current code will break such use case:
2 fixed IPs behind same router both have floatingip associated. And
the router has enable_snat=false. When fixed IP A want to ping
fixed IP B, fixed IP A will get the reply from fixed IP B's floating
IP.

More details could be found at bug description.

Change-Id: I322e8d454ef1d529ceda541fb5fe577cd70b412f
Closes-bug: #1505781