Add zones ownership transfer request to Designate tempest plugin

This patch adds zone transfer_request_client's methods and tests
to Designate tempest plugin.

Partially-Implements: blueprint designate-tempest-plugin

Change-Id: I0c5b4a5796c398fb1d24cde124203c865ea21833
diff --git a/designate_tempest_plugin/clients.py b/designate_tempest_plugin/clients.py
index 17df788..5cdb39e 100644
--- a/designate_tempest_plugin/clients.py
+++ b/designate_tempest_plugin/clients.py
@@ -38,6 +38,8 @@
     TldClient
 from designate_tempest_plugin.services.dns.query.query_client import \
     QueryClient
+from designate_tempest_plugin.services.dns.v2.json.transfer_request_client \
+    import TransferRequestClient
 
 CONF = config.CONF
 
@@ -92,3 +94,5 @@
             build_interval=CONF.dns.build_interval,
             build_timeout=CONF.dns.build_timeout,
         )
+        self.transfer_request_client = TransferRequestClient(
+                                           self.auth_provider, **params)