Prevent setting tempest fixed_network_name for cells

Instead of creating a network in both the API cell and child cell,
let tempest use the old behavior of not querying networks from the
API for testing server create.

Change-Id: I9809d2b2e796ff1a5ea7e4f25bbeb21bd4817a72
diff --git a/lib/tempest b/lib/tempest
index cd8fbd7..dc5fb51 100644
--- a/lib/tempest
+++ b/lib/tempest
@@ -340,7 +340,9 @@
     iniset $TEMPEST_CONFIG compute flavor_ref $flavor_ref
     iniset $TEMPEST_CONFIG compute flavor_ref_alt $flavor_ref_alt
     iniset $TEMPEST_CONFIG compute ssh_connect_method $ssh_connect_method
-    iniset $TEMPEST_CONFIG compute fixed_network_name $PRIVATE_NETWORK_NAME
+    if ! is_service_enabled n-cell; then
+        iniset $TEMPEST_CONFIG compute fixed_network_name $PRIVATE_NETWORK_NAME
+    fi
 
     # Compute Features
     # Run ``verify_tempest_config -ur`` to retrieve enabled extensions on API endpoints