Added time import in double tagged example

Change-Id: I3521200767e2533f75835fa287017ae28a1dbc5b
diff --git a/trellis/trellis_double_tagged.py b/trellis/trellis_double_tagged.py
index 6509e93..ac7e751 100755
--- a/trellis/trellis_double_tagged.py
+++ b/trellis/trellis_double_tagged.py
@@ -15,6 +15,7 @@
 from trellislib import DhcpClient, Dhcp6Client, DhcpRelay, DhcpServer, Dhcp6Server, DoubleTaggedRoutedHost
 from trellislib import get_mininet, parse_trellis_args, set_up_zebra_config
 from functools import partial
+from time import sleep
 
 class Trellis( Topo ):
     "Trellis basic topology"