Add tld_client's methods and tests to Designate tempest plugin

Partially-Implements: blueprint designate-tempest-plugin

Change-Id: Ic23a9249d0cf5c34e5c2c083b465200fa6c5da66
diff --git a/designate_tempest_plugin/data_utils.py b/designate_tempest_plugin/data_utils.py
index d3c72cc..f18e87c 100644
--- a/designate_tempest_plugin/data_utils.py
+++ b/designate_tempest_plugin/data_utils.py
@@ -206,3 +206,10 @@
     ns_records = [{"hostname": x, "priority": random.randint(1, 999)}
                   for x in records]
     return ns_records
+
+
+def rand_tld():
+    data = {
+        "name": rand_zone_name(prefix='tld', suffix='')
+    }
+    return data