Use 'route' mode for libvirt external network instead of 'nat'

Change for static pike/ocata virtual lab models

Change-Id: I256345038eade428db5830f4160256f632d7398f
diff --git a/tcp_tests/templates/cookied-mcp-mitaka-dvr/underlay.yaml b/tcp_tests/templates/cookied-mcp-mitaka-dvr/underlay.yaml
index 08f443e..7225c6d 100644
--- a/tcp_tests/templates/cookied-mcp-mitaka-dvr/underlay.yaml
+++ b/tcp_tests/templates/cookied-mcp-mitaka-dvr/underlay.yaml
@@ -162,7 +162,7 @@
             address_pool: external-pool01
             dhcp: true
             forward:
-              mode: nat
+              mode: route
 
 
         group_volumes: