Add ability to set custom internal dns zone name

Due to internal dns zone name can be custom
we should set it via config option.

Related-prod: PRODX-2967
Change-Id: I92f598c3b3035924e9b90820d0aab0d24dc9fb4e
(cherry picked from commit b866ad3d34fbe7f89b120359763fcca4281173bb)
diff --git a/neutron_tempest_plugin/config.py b/neutron_tempest_plugin/config.py
index 28d6b76..7d324dc 100644
--- a/neutron_tempest_plugin/config.py
+++ b/neutron_tempest_plugin/config.py
@@ -129,6 +129,9 @@
                      'If True, multicast test(s) will assert that multicast '
                      'traffic is not being flooded to all ports. Defaults '
                      'to False.'),
+    cfg.StrOpt('dns_domain',
+               default='openstackgate.local',
+               help='Name of internal dns zone to use in tests. '),
 ]
 
 # TODO(amuller): Redo configuration options registration as part of the planned