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-mcp-ocata-ceph-offline/underlay.yaml b/tcp_tests/templates/virtual-mcp-ocata-ceph-offline/underlay.yaml
index 49445ae..1fd9b17 100644
--- a/tcp_tests/templates/virtual-mcp-ocata-ceph-offline/underlay.yaml
+++ b/tcp_tests/templates/virtual-mcp-ocata-ceph-offline/underlay.yaml
@@ -165,7 +165,7 @@
             address_pool: external-pool01
             dhcp: true
             forward:
-              mode: nat
+              mode: route
 
         group_volumes:
          - name: cloudimage1604    # This name is used for 'backing_store' option for node volumes.