[contrail][horizon] Bad subnet request: update of allocation_pools is not allowed

Detailed bug description:
The error appears when horizon user updates DNS servers addresses in existing subnet (VN subnet).

Erros comes from Contrail, log files contains

in Contrail controller /var/log/contrail/contrail-api-0-stdout.log
172.18.62.1 - - [2016-11-01 13:32:00] "POST /neutron/subnet HTTP/1.1" 200 559 0.127021
<pre>Traceback (most recent call last):
  File "/usr/lib/python2.7/dist-packages/vnc_openstack/neutron_plugin_interface.py", line 290, in plugin_update_subnet
    subnet['resource'])
  File "/usr/lib/python2.7/dist-packages/vnc_openstack/neutron_plugin_db.py", line 2325, in wrapper
    return func(self, *args, **kwargs)
  File "/usr/lib/python2.7/dist-packages/vnc_openstack/neutron_plugin_db.py", line 2659, in subnet_update
    msg="update of allocation_pools is not allowed")
  File "/usr/lib/python2.7/dist-packages/vnc_openstack/neutron_plugin_db.py", line 194, in _raise_contrail_exception
    bottle.abort(400, json.dumps(exc_info))
  File "/usr/lib/python2.7/dist-packages/bottle.py", line 2310, in abort
    raise HTTPError(code, text)
HTTPError
</pre>
172.18.62.1 - - [2016-11-01 13:32:01] "POST /neutron/subnet HTTP/1.1" 400 227 0.007812

in MOS controller /var/log/neutron/server.log:2016-11-01 13:45:45.400 24899 INFO neutron.api.v2.resource [req-e88a1d62-cb23-413a-98c4-abb5c48a1536 805a89456ccbbe3c338a1472d0127b3b3b01133a75d53e6246eee2f49aefdf20 20446d16f9ef4613871ef4272fe8e12c - - -] update failed (client error): Bad subnet request: update of allocation_pools is not allowed

in MOS controller /var/log/horizon/horizon.log
2016-11-01 12:50:31,803 1260 WARNING horizon.exceptions Recoverable error: Bad subnet request: update of allocation_pools is not allowed

Updating DNS servers using CLI works just fine:

root@node-22:/etc/cinder# neutron subnet-show a662c998-1080-4c03-8fb9-2ae69caa99f7
+-------------------+------------------------------------------------+
| Field | Value |
+-------------------+------------------------------------------------+
| allocation_pools | {"start": "10.10.10.10", "end": "10.10.10.32"} |
| cidr | 10.10.10.0/24 |
| dns_nameservers | 8.8.8.8 |
| enable_dhcp | True |
| gateway_ip | 10.10.10.1 |
| host_routes | |
| id | a662c998-1080-4c03-8fb9-2ae69caa99f7 |
| ip_version | 4 |
| ipv6_address_mode | |
| ipv6_ra_mode | |
| name | Joomla subnet |
| network_id | 62038f1a-904b-46a7-926b-562e3162a646 |
| tenant_id | 3f6dcba833ff42a5a10541afdadc0da6 |
+-------------------+------------------------------------------------+
root@node-22:/etc/cinder# neutron subnet-update --dns-nameserver 4.4.2.2 a662c998-1080-4c03-8fb9-2ae69caa99f7
Updated subnet: a662c998-1080-4c03-8fb9-2ae69caa99f7
root@node-22:/etc/cinder# neutron subnet-show a662c998-1080-4c03-8fb9-2ae69caa99f7
+-------------------+------------------------------------------------+
| Field | Value |
+-------------------+------------------------------------------------+
| allocation_pools | {"start": "10.10.10.10", "end": "10.10.10.32"} |
| cidr | 10.10.10.0/24 |
| dns_nameservers | 4.4.2.2 |
| enable_dhcp | True |
| gateway_ip | 10.10.10.1 |
| host_routes | |
| id | a662c998-1080-4c03-8fb9-2ae69caa99f7 |
| ip_version | 4 |
| ipv6_address_mode | |
| ipv6_ra_mode | |
| name | Joomla subnet |
| network_id | 62038f1a-904b-46a7-926b-562e3162a646 |
| tenant_id | 3f6dcba833ff42a5a10541afdadc0da6 |
+-------------------+------------------------------------------------+

Steps to reproduce:
deploy MOS8 with fuel-plugin-contrail 4.0, go to Horizon Network->Networks
select some VN, edit existing subnetwork, go to Subnet Detail tab change 'DNS Name Servers' value, hit 'Create'

Expected results:
 DNS servers has been changed
Actual result:
 error - Bad subnet request: update of allocation_pools is not allowed

Description of the environment:
VERSION:
  feature_groups:
    - mirantis
  production: "docker"
  release: "8.0"
  api: "1.0"
  build_number: "570"
  build_id: "570"
  fuel-nailgun_sha: "558ca91a854cf29e395940c232911ffb851899c1"
  python-fuelclient_sha: "4f234669cfe88a9406f4e438b1e1f74f1ef484a5"
  fuel-agent_sha: "658be72c4b42d3e1436b86ac4567ab914bfb451b"
  fuel-nailgun-agent_sha: "b2bb466fd5bd92da614cdbd819d6999c510ebfb1"
  astute_sha: "b81577a5b7857c4be8748492bae1dec2fa89b446"
  fuel-library_sha: "c2a335b5b725f1b994f78d4c78723d29fa44685a"
  fuel-ostf_sha: "3bc76a63a9e7d195ff34eadc29552f4235fa6c52"
  fuel-mirror_sha: "fb45b80d7bee5899d931f926e5c9512e2b442749"
  fuelmenu_sha: "78ffc73065a9674b707c081d128cb7eea611474f"
  shotgun_sha: "63645dea384a37dde5c01d4f8905566978e5d906"
  network-checker_sha: "a43cf96cd9532f10794dce736350bf5bed350e9d"
  fuel-upgrade_sha: "616a7490ec7199f69759e97e42f9b97dfc87e85b"
  fuelmain_sha: "d605bcbabf315382d56d0ce8143458be67c53434"

[root@fuel8 ~]# fuel plugins | grep contrail
16 | contrail | 4.0.0 | 4.0.0

# contrail-version
Package Version Build-ID | Repo | Package Name
-------------------------------------- ------------------------------ ----------------------------------
contrail-analytics 3.0.2.0-51 51
contrail-config 3.0.2.0-51 51
contrail-config-openstack 3.0.2.0-51 51
contrail-control 3.0.2.0-51 51
contrail-dns 3.0.2.0-51 51
contrail-f5 3.0.2.0-51 51
contrail-lib 3.0.2.0-51 51
contrail-nodemgr 3.0.2.0-51 51
contrail-openstack-analytics 3.0.2.0-51 51
contrail-openstack-config 3.0.2.0-51 51
contrail-openstack-control 3.0.2.0-51 51
contrail-openstack-database 3.0.2.0-51 51
contrail-openstack-webui 3.0.2.0-51 51
contrail-setup 3.0.2.0-51 51
contrail-utils 3.0.2.0-51 51
contrail-web-controller 3.0.2.0-51 51
contrail-web-core 3.0.2.0-51 51
ifmap-python-client 0.1-2 51
ifmap-server 0.3.2-1contrail2 51
neutron-plugin-contrail 3.0.2.0-51 51
python-contrail 3.0.2.0-51 51
python-neutronclient 1:3.1.0-0ubuntu1~cloud0.2contrail51