While creating/updating a neutron subnet user can specify dnsNameservers as list. NeutronNorthbound doesn't store dnsNameservers information in MDSAL even though yang for it exists.

Steps to reproduce:

1. If using openstack setup, use following command to create subnet with dnsNameservers:
neutron subnet-create --dns-nameserver 8.8.8.8 --name net1-snet1

2. If using Curl/Postman, use the following JSON:
{
"subnet": {
"name": "",
"enable_dhcp": true,
"network_id": "e9330b1f-a2ef-4160-a991-169e56ab17f5",
"tenant_id": "4fd44f30292945e481c7b8a0c8908869",
"dns_nameservers": [
"8.8.8.8"
],
"allocation_pools": [

{ "start": "192.168.199.2", "end": "192.168.199.254" }

],
"ip_version": 4,
"gateway_ip": "192.168.199.1",
"cidr": "192.168.199.0/24",
"id": "3b80198d-4f7b-4f77-9ef5-774d54e17126"
}
}

Expected result: dnsNameservers should be stored in MDSAL.
Actual result: dnsNameservers is an empty list in MDSAL.

This impacts VPNservice which uses this information when a VM comes up.