After disabling snat for router enabling it fails

The fix is not merged upstream yet, so we are waiting for its resolution.

Fix proposed to branch: 9.0/mitaka
Change author: Rodion Tikunov <email address hidden>
Review: https://review.fuel-infra.org/28344

For reproducing.
Before steps from bug description, do two more steps:
1) Set enable_snat_by_default = false in /etc/neutron/neutron.conf
2) service neutron-server restart

Series of patches to stable branches is on the review in Upstream.

https://review.openstack.org/#/q/I01009d5cd5edd5be3eead615c37d6aa2e3224442,n,z

Fixed by https://review.fuel-infra.org/#/c/29104/

Change abandoned by Rodion Tikunov <email address hidden> on branch: 9.0/mitaka
Review: https://review.fuel-infra.org/28344
Reason: Synced from upstream

Verified on 9.2 snapshot #593.
After disabling snat for router gateway, enabling it back complete successfully without any error.

Fixed by https://review.fuel-infra.org/#/c/29066/ for 10.0/newton

Fix proposed to branch: openstack-ci/fuel-7.0/2015.1.0
Change author: Rodion Tikunov <email address hidden>
Review: https://review.fuel-infra.org/35143

Reviewed: https://review.fuel-infra.org/35143
Submitter: Denis V. Meltsaykin <email address hidden>
Branch: openstack-ci/fuel-7.0/2015.1.0

Commit: cf1e6fcba50b25200ce9b55596d06f36aefeb544
Author: Rodion Tikunov <email address hidden>
Date: Wed May 17 12:27:18 2017

Added --enable-snat option for router-gateway-set

If enable_snat_by_default option set to false and disable snat via cli
it becomes unavailable to enable snat again.
This commit allows to enable snat after disabling it.

Change-Id: I01009d5cd5edd5be3eead615c37d6aa2e3224442
Closes-Bug: #1626442
(cherry picked from commit cc1d3fdd3582b0bbc2e24b65b9c690b5f0318148)

Verified on 7.0 mu8 updates.

After disabling snat for router gateway, enabling it back complete successfully without any error.

Patch for 8.0 branch (openstack-ci/fuel-8.0/liberty) was cherry_picked from upstream 'stable/mitaka ' branch:
https://review.fuel-infra.org/#/c/35626/

It was merged.

Verified on Fuel 8.0 MU-5

root@node-1:~# neutron router-list
+--------------------------------------+----------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+-------------+-------+
| id | name | external_gateway_info | distributed | ha |
+--------------------------------------+----------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+-------------+-------+
| d59c36b4-c7ca-4ce4-999d-ee67c3b5bb39 | router04 | {"network_id": "e105957e-0a29-498a-9291-f94bdf5a60cb", "enable_snat": true, "external_fixed_ips": [{"subnet_id": "1df84671-cd94-4eaa-8334-c5f99ad7be19", "ip_address": "10.109.8.128"}]} | False | False |
+--------------------------------------+----------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+-------------+-------+
root@node-1:~#
root@node-1:~#
root@node-1:~#
root@node-1:~# neutron router-gateway-set --disable-snat d59c36b4-c7ca-4ce4-999d-ee67c3b5bb39 e105957e-0a29-498a-9291-f94bdf5a60cb
Set gateway for router d59c36b4-c7ca-4ce4-999d-ee67c3b5bb39
root@node-1:~#
root@node-1:~# neutron router-list
+--------------------------------------+----------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+-------------+-------+
| id | name | external_gateway_info | distributed | ha |
+--------------------------------------+----------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+-------------+-------+
| d59c36b4-c7ca-4ce4-999d-ee67c3b5bb39 | router04 | {"network_id": "e105957e-0a29-498a-9291-f94bdf5a60cb", "enable_snat": false, "external_fixed_ips": [{"subnet_id": "1df84671-cd94-4eaa-8334-c5f99ad7be19", "ip_address": "10.109.8.128"}]} | False | False |
+--------------------------------------+----------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+-------------+-------+
root@node-1:~#
root@node-1:~#
root@node-1:~#
root@node-1:~# neutron router-gateway-set --enable-snat d59c36b4-c7ca-4ce4-999d-ee67c3b5bb39 e105957e-0a29-498a-9291-f94bdf5a60cb
Set ga...