Use "ip neigh" instead of "arp -n" in worlddump.py

Worlddumping on a system without net-tools package gets an error showing
"arp: not found". As iproute2 can also show arp tables, we use it
instead.

Change-Id: I0cd83e6d14959dc5a1147c487b11f27fb92aa20a
diff --git a/tools/worlddump.py b/tools/worlddump.py
index 6fff149..7506082 100755
--- a/tools/worlddump.py
+++ b/tools/worlddump.py
@@ -164,8 +164,7 @@
     _header("Network Dump")
 
     _dump_cmd("brctl show")
-    _dump_cmd("arp -n")
-    ip_cmds = ["addr", "link", "route"]
+    ip_cmds = ["neigh", "addr", "link", "route"]
     for cmd in ip_cmds + ['netns']:
         _dump_cmd("ip %s" % cmd)
     for netns_ in _netns_list():