feat: provide a working default `network-config` for cloud-init.

[ajf]

In #3392, #2814, and #3303 a bunch of cloud-init changes happened, resulting in breaking user-data when using qcow-imager as its using files downloaded from NICo and copied into nocloud-net. The fix in #3392 extracted network key from the user-data and wrote it to network_config cloud init.

cloud-init docs:

User-data cannot change an instance’s network configuration. In the absence of network configuration in any of the above sources, cloud-init will write out a network configuration that will issue a DHCP request on a “first” network interface.

Probably in the future we can provide a default network_config that DHCPs all interfaces (the default for cloud-init is first network interface only)