DHCP client: add an option to disable hostname request

[smira]

OpenStack platform provides network configuration with the hostname and enables DHCP for eth0.

Talos prefers operator-based (e.g. DHCP) resources over platform resources, so DHCP hostname overrides OpenStack (platform) hostname.

To fix this issue, we should offer an option for DHCP to skip sending hostname request (and not setting hostname via DHCP at all).

OpenStack platform should set this option when a link is configured for DHCP, but OpenStack platform provides its own hostname.

[smira]

(it's not required to expose this in the machine config)

[smira]

sample network config:

"network_json": {
   "links": [
    {
     "ethernet_mac_address": "fa:16:3e:7e:0f:51",
     "id": "tapc5a8eea1-eb",
     "mtu": 1450,
     "type": "ovs",
     "vif_id": "c5a8eea1-eb06-4c0e-9de3-4a1f39d5a26c"
    }
   ],
   "networks": [
    {
     "id": "network0",
     "link": "tapc5a8eea1-eb",
     "network_id": "d8b16a08-5995-49bb-8904-5ae2cde882c1",
     "type": "ipv4_dhcp"
    }
   ],
   "services": [
    {
     "address": "10.171.10.1",
     "type": "nameserver"
    },
    {
     "address": "10.171.10.2",
     "type": "nameserver"
    }
   ]
  }