nixos/tests/taskserver: fix eval

Since IP address options were changed for 18.03, eval has failed with:
"The option `networking.interfaces.eth1.subnetMask' is used but not defined."
although this option is not used at all in nixos anymore.

The misleading error message seems to be generated from evaluating warnings
for `mkRemovedOptionModule ["subnetMask"]` which apparently broke here
when this test inherited network.interfaces from one VM config to another.

Cc: @aszlig

nixos/tests/taskserver.nix

@@ -82,8 +82,9 @@ in {
       # This is to avoid assigning a different network address to the new
       # generation.
       networking = lib.mapAttrs (lib.const lib.mkForce) {
+        interfaces.eth1.ipv4 = nodes.server.config.networking.interfaces.eth1.ipv4;
         inherit (nodes.server.config.networking)
-          hostName interfaces primaryIPAddress extraHosts;
+          hostName primaryIPAddress extraHosts;
       };
     };