cloud-init fails with KeyError when nameserver config and matching interface are present

cloud-init fails to parse network configuration with a KeyError if:

- The configuration has a `nameservers` configuration, and
- The configuration matches an existing interface on the machine, and
- The configuration name does not match that interfaces name.

Consider the following network config:

```
version: 2
ethernets:
  eth:
    match:
      macaddress: '00:11:22:33:44:55'
    addresses: [10.0.0.2/24]
    gateway4: 10.0.0.1
    nameservers:
      addresses: [10.0.0.1]
```

Now, if a device is present with the mac address 00:11:22:33:44:55, parsing will fail with the following stack trace:

```
2022-06-25 10:26:43,657 - util.py[WARNING]: failed stage init-local
2022-06-25 10:26:43,657 - util.py[DEBUG]: failed stage init-local
Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/cloudinit/cmd/main.py", line 738, in status_wrapper
    ret = functor(name, args)
  File "/usr/lib/python3/dist-packages/cloudinit/cmd/main.py", line 410, in main_init
    init.apply_network_config(bring_up=bring_up_interfaces)
  File "/usr/lib/python3/dist-packages/cloudinit/stages.py", line 937, in apply_network_config
    return self.distro.apply_network_config(
  File "/usr/lib/python3/dist-packages/cloudinit/distros/__init__.py", line 231, in apply_network_config
    network_state = parse_net_config_data(netconfig)
  File "/usr/lib/python3/dist-packages/cloudinit/net/network_state.py", line 1056, in parse_net_config_data
    nsi.parse_config(skip_broken=skip_broken)
  File "/usr/lib/python3/dist-packages/cloudinit/net/network_state.py", line 278, in parse_config
    self.parse_config_v2(skip_broken=skip_broken)
  File "/usr/lib/python3/dist-packages/cloudinit/net/network_state.py", line 328, in parse_config_v2
    self._v2_common(command)
  File "/usr/lib/python3/dist-packages/cloudinit/net/network_state.py", line 782, in _v2_common
    self._handle_individual_nameserver(name_cmd, iface)
  File "/usr/lib/python3/dist-packages/cloudinit/net/network_state.py", line 110, in decorator
    return func(self, command, *args, **kwargs)
  File "/usr/lib/python3/dist-packages/cloudinit/net/network_state.py", line 570, in _handle_individual_nameserver
    _iface[iface]["dns"] = {"nameservers": nameservers, "search": search}
KeyError: 'eth'
```

I've investigated the issue myself and created a test case:

```
from unittest import mock

from cloudinit import safeyaml
from cloudinit.net import network_state

class TestNetDns:
    @mock.patch("cloudinit.net.network_state.get_interfaces_by_mac")
    def test_networkd_render_x(self, by_mac):
        by_mac.return_value = {"00:11:22:33:44:55": "foobar"}
        network_state.parse_net_config_data(safeyaml.load("""\
version: 2
ethernets:
  eth:
    match:
      macaddress: '00:11:22:33:44:55'
    addresses: [10.0.0.2/24]
    gateway4: 10.0.0.1
    nameservers:
      addresses: [10.0.0.1]
"""))
```

This test case will fail with the KeyError.

The reason for this bug is that `handle_ethernets` will take the interface name from the system because no set-name setting is present. It will then add the network state under that interface name, not the configured key 'eth'. Later, _handle_individual_nameserver will try to look up the state for 'eth' and fail.

A quick bisect shows that this test case starts failing with commit bf94945fb855c40c5188cef5fb00327c51c41fef (though the mock doesn't work very far in the past). This commit introduced the name handling logic.

As a workaround, you can give an explicit set-name statement, or change the key of the network config to match the physical device name.