Fixes that `containers.<name>.extraVeths.<name>` configuration was not
always applied.
When configuring `containers.<name>.extraVeths.<name>` and not
configuring one of `containers.<name>.localAddress`, `.localAddress6`,
`.hostAddress`, `.hostAddress6` or `.hostBridge` the veth was created,
but otherwise no configuration (i.e. no ip) was applied.
nixos-container always configures the primary veth (when `.localAddress`
or `.hostAddress` is set) to be the containers default gateway, so
this fix is required to create a veth in containers that use a different
default gateway.
To test this patch configure the following container and check if the
addresses are applied:
```
containers.testveth = {
extraVeths.testveth = {
hostAddress = "192.168.13.2";
localAddress = "192.168.13.1";
};
config = {...}:{};
};
```
This follows upstreams change in documentation. While the `[DHCP]`
section might still work it is undocumented and we should probably not
be using it anymore. Users can just upgrade to the new option without
much hassle.
I had to create a bit of custom module deprecation code since the usual
approach doesn't support wildcards in the path.
