nuageinit: config2_network support dns service cfg

This change enhances config2_network() to honor the DNS
configuration, when it's defined through the services section.

The network_data.json file can hold DNS configuration at two different
places:

• within a network configuration entry
• or dns entry in the services section, in this case the configuration is global.

An example of such configuration:

{"links": [{"id": "interface0", "type": "phy",
"ethernet_mac_address": "52:54:00:01:59:03"}], "networks": [{"id": "private-ipv4-0", "type": "ipv4", "link": "interface0",
"ip_address": "192.168.123.5", "netmask": "255.255.255.0", "routes": [{"network": "0.0.0.0", "netmask": "0.0.0.0", "gateway":
"192.168.123.1"}], "network_id": "9e5b1ed9-f5e6-4941-a90f-2e06bab858de", "dns_nameservers": ["192.168.123.1"], "services": [{"type":
"dns", "address": "192.168.123.1"}]}], "services": [{"type": "dns", "address": "192.168.123.1"}]}

See: https://docs.openstack.org/nova/latest/user/metadata.html

MFC After: 1 week
Signed-off-by: Gonéri Le Bouder <goneri@lebouder.net>
Pull Request: https://github.com/freebsd/freebsd-src/pull/1941
(cherry picked from commit 7af8b75201b7af353fa74c8feb937b52f5ccf9bb)