[backport 1443524] Removing an interface by port from a DVR router deletes all SNAT ports

[Oleg Bondarev]

This is a backport for https://bugs.launchpad.net/neutron/+bug/1443524

Original description
================

Steps to reproduce:
0) Multinode setup
1) Create distributed router
2) Add two internal interfaces
3) Set the router as gateway

At this point the router has 5 ports: Two internal, one external, and two owned by DVR SNAT

4) Now delete the first internal interface (By port, not by subnet).

Three ports were deleted instead of two: The internal port, its SNAT port, and the SNAT port of another interface.

[Fuel Devops McRobotson (fuel-devops-robot)]

Fix proposed to branch: openstack-ci/fuel-7.0/2015.1.0
Change author: Oleg Bondarev <email address hidden>
Review: https://review.fuel-infra.org/10500

[Fuel Devops McRobotson (fuel-devops-robot)]

Reviewed: https://review.fuel-infra.org/10500
Submitter: mos-infra-ci <>
Branch: openstack-ci/fuel-7.0/2015.1.0

Commit: 8d234c41109d24dce32315e10108344482151ca5
Author: Oleg Bondarev <email address hidden>
Date: Mon Aug 17 16:19:27 2015

Fix DVR interface delete by port when gateway is set

When removing a DVR interface by port, the subnet_id
passed to delete_csnat_router_interface_ports is None,
and so it deletes all the DVR SNAT ports for the
router.
This patch fixes this issue by passing in the right
subnet_id to the delete_csnat_router_interface_ports.

upstream review: https://review.openstack.org/173001

Closes-Bug: #1485658
Closes-Bug: #1443524
Change-Id: I16735195c6575454876acd0e99ef45f382963566