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

change for virtual cookied labs for now.

Change-Id: I01a24a539b993515ae346f7a79d87feba3685dbe
diff --git a/tcp_tests/templates/cookied-mcp-ocata-dop-sl2/underlay.yaml b/tcp_tests/templates/cookied-mcp-ocata-dop-sl2/underlay.yaml
index fbbda4b..77b2573 100644
--- a/tcp_tests/templates/cookied-mcp-ocata-dop-sl2/underlay.yaml
+++ b/tcp_tests/templates/cookied-mcp-ocata-dop-sl2/underlay.yaml
@@ -212,7 +212,7 @@
             address_pool: external-pool01
             dhcp: false
             forward:
-              mode: nat
+              mode: route
 
 
         group_volumes: