nixosTests.keepalived: fix eval (maintainers attribute)

Without the change test sfails to evaluate as:

    $ nix build --no-link -f. nixosTests.keepalived
    error:
       error: function 'makeTest' called with unexpected argument 'maintainers'

nixos/tests/keepalived.nix

@@ -1,6 +1,6 @@
 import ./make-test-python.nix ({ pkgs, lib, ... }: {
   name = "keepalived";
-  maintainers = [ lib.maintainers.raitobezarius ];
+  meta.maintainers = [ lib.maintainers.raitobezarius ];
 
   nodes = {
     node1 = { pkgs, ... }: {