Use fixed network for ssh when n-net is enabled

Server rebuild test has stability issues with n-net + floating ip.

Change-Id: I8a921fddbca49c8499938a25f9722ea40cee76cc
diff --git a/lib/tempest b/lib/tempest
index 803b740..5a2c78d 100644
--- a/lib/tempest
+++ b/lib/tempest
@@ -193,7 +193,9 @@
 
     if [ "$Q_USE_NAMESPACE" != "False" ]; then
         tenant_networks_reachable=false
-        ssh_connect_method="floating"
+        if ! is_service_enabled n-net; then
+            ssh_connect_method="floating"
+        fi
     else
         tenant_networks_reachable=true
     fi