Catch PortNotFound exception during get_dhcp_port

Close-bug: 1252437

Change-Id: I3a7954b869b4f5b909c806f8e47180e1ba6ba374
(cherry picked from commit cbe91fda7b416717a23cf7b29adf8201a8c14822)
Merge "Catch PortNotFound exception during get_dhcp_port" into stable/havana
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 remote-tracking branch 'upstream/stable/havana' into stable/havana-shashank