Description of problem:
-----------------------
Given there is an existing OVN IPv6 subnet entity on the oVirt-provider-OVN.
When I attempt to update the subnet's 'ipv6_address_mode' parameter to a different value
Then the provider returns [200] 'OK' when in fact nothing changes.

Example: 
(updating the 'ipv6_address_mode' parameter from 'dhcpv6_stateful' to dhcpv6_stateless')
-------
[BEFORE UPDATE]
{
    "subnet": {
        "allocation_pools": [
            {
                "start": "def0:4141:ab53::2",
                "stop": "def0:4141:ab53:0:ffff:ffff:ffff:ffff"
            }
        ],
        "ipv6_address_mode": "dhcpv6_stateful",
        "cidr": "def0:4141:ab53::/64",
        "id": "02441ffd-4072-4877-8489-c18527c6ec79",
        "name": "moshe_ovn_ipv6_subnet",
        "enable_dhcp": true,
        "network_id": "5fb6e896-8a6d-4657-be93-847549662952",
        "tenant_id": "00000000000000000000000000000001",
        "dns_nameservers": [],
        "gateway_ip": "def0:4141:ab53::1",
        "ip_version": 6
    }
}

[UPDATE REQUEST]
PUT {{oVirt_fqdn}}:9696/v2.0/subnets/{{subnet_ID}}
body:
{
    "subnet": {
        "allocation_pools": [
            {
                "start": "def0:4141:ab53::2",
                "stop": "def0:4141:ab53:0:ffff:ffff:ffff:ffff"
            }
        ],
        "ipv6_address_mode": "dhcpv6_stateless",
        "cidr": "def0:4141:ab53::/64",
        "id": "02441ffd-4072-4877-8489-c18527c6ec79",
        "name": "moshe_ovn_ipv6_subnet",
        "enable_dhcp": true,
        "network_id": "5fb6e896-8a6d-4657-be93-847549662952",
        "tenant_id": "00000000000000000000000000000001",
        "dns_nameservers": [],
        "gateway_ip": "def0:4141:ab53::1",
        "ip_version": 6
    }
}

[RESPONSE]
[200] OK
body:
{
    "subnet": {
        "allocation_pools": [
            {
                "start": "def0:4141:ab53::2",
                "stop": "def0:4141:ab53:0:ffff:ffff:ffff:ffff"
            }
        ],
        "ipv6_address_mode": "dhcpv6_stateful",
        "cidr": "def0:4141:ab53::/64",
        "id": "02441ffd-4072-4877-8489-c18527c6ec79",
        "name": "moshe_ovn_ipv6_subnet",
        "enable_dhcp": true,
        "network_id": "5fb6e896-8a6d-4657-be93-847549662952",
        "tenant_id": "00000000000000000000000000000001",
        "dns_nameservers": [],
        "gateway_ip": "def0:4141:ab53::1",
        "ip_version": 6
    }
}

Version-Release number of selected component (if applicable):
---------------------------------------------------------------
ovirt-provider-ovn-1.2.22-0.20190424100824.gita51fe76.el7.noarch
ovirt-engine-4.4.0-0.0.master.20190416073615.gitba51258.el7.noarch
How reproducible:


Steps to Reproduce:
------------------
1. Create an IPv6 subnet
2. Update the 'ipv6_address_mode' parameter

Actual results:
----------------
The provider returns an OK response but does not update anything

Expected results:
-----------------
An error should be returned by the provider indicating this parameter cannot be changed once the subnet is created