Rectify test of bgp l3vpn

Related-Bug: PRODX-11809
Change-Id: Ifa2f905000a4e8123e7905af4faa8fc94d1ee064
diff --git a/neutron_tempest_plugin/config.py b/neutron_tempest_plugin/config.py
index 9917817..bac18f9 100644
--- a/neutron_tempest_plugin/config.py
+++ b/neutron_tempest_plugin/config.py
@@ -165,6 +165,10 @@
                default=200,
                help=("Maximum number for the range of "
                      "assigned number for distinguishers.")),
+    cfg.StrOpt('l3vpn_private_subnet',
+               default='10.22.0.0/24',
+               help=("IP Address/Mask of instance, which will be "
+                     "used to test the l3vpn connectivity.")),
     cfg.StrOpt('l3vpn_endpoint',
                default=None,
                help=("IP Address/Mask, which will be used to "