NVP plugin:fix connectivity to fip from internal nw

The current set of NAT rules configured in the NVP logical router
prevents instances on an internal network from establishing TCP
connections with a floating IP associate with an internal port
on the same Neutron network.

This patch add a SNAT rule to ensure connections coming from the
internal network always appear to the port associate with the
floating IP as coming from the external network.

Change-Id: I12da12ee136c61fddb04f2b216382b51d751a02c
Closes-Bug: 1221419
(cherry picked from commit ed2e32b03123ae8e7555b997152c1d8d941494f2)
Merge branch 'stable/havana' of https://github.com/openstack/quantum into havana/stable