Skip TrunkTestMtusJSONBase class tests when provider_vlans is not defined

Related-Prod: PRODX-47802
Change-Id: Ibd918c1e1acea3bdb0287cfd91473d43b84136f1
diff --git a/neutron_tempest_plugin/api/test_trunk.py b/neutron_tempest_plugin/api/test_trunk.py
index 26f8de8..3867f2d 100644
--- a/neutron_tempest_plugin/api/test_trunk.py
+++ b/neutron_tempest_plugin/api/test_trunk.py
@@ -250,6 +250,8 @@
         if not all(cls.is_type_driver_enabled(t) for t in ['vlan', 'vxlan']):
             msg = "Either vxlan or vlan type driver not enabled."
             raise cls.skipException(msg)
+        if not CONF.neutron_plugin_options.provider_vlans:
+            raise cls.skipException("No provider VLAN networks available")
 
     def setUp(self):
         super(TrunkTestMtusJSONBase, self).setUp()