tempest: conditional --provider:physical_network

Juniper Contrail associate the use of `--provider:physical_network` to a
SRIOV device. Which means the parameter cannot be used seamlessly.
With this patch, we use the argument only if `public_physical_network` is
not empty.

See: https://www.juniper.net/documentation/en_US/contrail3.1/topics/concept/sriov-with-vrouter-vnc.html

Change-Id: I983cc88f6f31ebb5deabbc8c67a3ec31f48b0f3d

roles/validate-tempest/README.md

@@ -37,6 +37,7 @@ Role Variables
 * `tempest_conf_removal`: dict - A dict of tempest configuration which will be
                           removed from tempest.conf file.
                           Format: section.key: value
+* `public_physical_network`: <string> The name of the border physical network (default: datacentre).
 
 Skip tests file
 ---------------

roles/validate-tempest/templates/configure-tempest.sh.j2

@@ -42,11 +42,14 @@ for i in $(neutron port-list -c id -f value); do neutron port-delete $i; done
 for i in $(neutron net-list -c id -f value); do neutron net-delete $i; done
 
 neutron net-create {{ public_net_name }} --router:external=True \
+{% if public_physical_network != '' %}
     --provider:network_type {{ public_network_type }} \
     {% if public_segmentation_id != '' %}
     --provider:segmentation_id {{ public_segmentation_id }} \
 {% endif %}
     --provider:physical_network {{ public_physical_network }}
+{% endif %}
+
 
 public_net_id=$(neutron net-show {{ public_net_name }} -f value -c id)