With that patch it's possible to deploy containers and assign those to
different IPs rather than the next available /24 subnet from `10.233.0.0/16`.
This patch depends on #60029[1] and can only be used with NixOS 19.09
(a.k.a nixos-unstable at the time of writing).

The behavior can be tested using a simple deployment like this:

``` nix
{
  container = { resources, ... }:
    { deployment.targetEnv = "container";
      deployment.container.host = resources.machines.horst;
      deployment.container.localAddress = "10.235.1.2";
      deployment.container.hostAddress = "10.235.1.1";
    };

  # the feature is optional, by default the next free addresses
  # from 10.233.0.0/16 will be used here.
  container2 = { resources, ... }:
    { deployment.targetEnv = "container";
      deployment.container.host = resources.machines.horst;
    };

  horst = { ... }:
    { deployment.targetEnv = "libvirtd";
    };
}
```

[1] NixOS/nixpkgs#60029