Fix sample conf for compatability with devstack.

Change-Id: I9411e3fd25ade2a09f2469306273cbaedf5ce910
diff --git a/etc/tempest.conf.sample b/etc/tempest.conf.sample
index 534f3d9..cfee55f 100644
--- a/etc/tempest.conf.sample
+++ b/etc/tempest.conf.sample
@@ -204,13 +204,13 @@
 tenant_networks_reachable = false
 
 # Id of the public network that provides external connectivity.
-public_network_id = {$PUBLIC_NETWORK_UUID}
+public_network_id = {$PUBLIC_NETWORK_ID}
 
 # Id of a shared public router that provides external connectivity.
 # A shared public router would commonly be used where IP namespaces
 # were disabled.  If namespaces are enabled, it would be preferable
 # for each tenant to have their own router.
-public_router_id =
+public_router_id = {$PUBLIC_ROUTER_ID}
 
 [network-admin]
 # This section contains configuration options for an administrative