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/virtual-offline-ssl/underlay.yaml b/tcp_tests/templates/virtual-offline-ssl/underlay.yaml
index 47b1c57..18c2bf1 100644
--- a/tcp_tests/templates/virtual-offline-ssl/underlay.yaml
+++ b/tcp_tests/templates/virtual-offline-ssl/underlay.yaml
@@ -160,7 +160,7 @@
             address_pool: external-pool01
             dhcp: true
             forward:
-              mode: nat
+              mode: route
 
 
         group_volumes: