replace ping6 with ping

reason: after the upgrade of iputils from 20151218 to 20161105
functionality of ping6 and tracepath6 was merged into ping and tracepath.

Ping is now mostly a drop-in replacment for ping6, except that selecting a
specific interface is done by encoding it into the address (ex.: fe80::1%eth0)
rather then specifing it with the `-I` flag.

nixos/tests/containers-extra_veth.nix

@@ -84,7 +84,7 @@ import ./make-test.nix ({ pkgs, ...} : {
 
       # Ping on main veth
       $machine->succeed("ping -n -c 1 192.168.0.100");
-      $machine->succeed("ping6 -n -c 1 fc00::2");
+      $machine->succeed("ping -n -c 1 fc00::2");
 
       # Ping on the first extra veth
       $machine->succeed("ping -n -c 1 192.168.1.100 >&2");