Make dns_domain value configurable

Test test_dns_domain_and_name uses a hardcoded dns_domain value, which
corresponds with the value configured in neutron repository.
This value could change and/or it can be different in other CI jobs.
Due to that, with this patch the dns_domain value becomes configurable.

Change-Id: Ib35b2ecd3a7350f5b744e5e063cfa25b1ecd4806
diff --git a/neutron_tempest_plugin/config.py b/neutron_tempest_plugin/config.py
index aea79ad..4fad1fa 100644
--- a/neutron_tempest_plugin/config.py
+++ b/neutron_tempest_plugin/config.py
@@ -73,6 +73,10 @@
                choices=['None', 'openvswitch', 'ovn',
                         'iptables_hybrid', 'iptables'],
                help='Driver for security groups firewall in the L2 agent'),
+    cfg.StrOpt('dns_domain',
+               default='openstackgate.local',
+               help='dns_domain value configured at neutron.conf, which will '
+                    'be used for the DNS configuration of the instances'),
 
     # Multicast tests settings
     cfg.StrOpt('multicast_group_range',