Add client's methods and testcases for tsigkey

Change-Id: I86cdf2b62f3586d35652891e4ace70ff8548640b
diff --git a/designate_tempest_plugin/clients.py b/designate_tempest_plugin/clients.py
index 9986a43..66db3de 100644
--- a/designate_tempest_plugin/clients.py
+++ b/designate_tempest_plugin/clients.py
@@ -42,6 +42,8 @@
     import TransferRequestClient
 from designate_tempest_plugin.services.dns.v2.json.transfer_accepts_client \
     import TransferAcceptClient
+from designate_tempest_plugin.services.dns.v2.json.tsigkey_client \
+    import TsigkeyClient
 
 CONF = config.CONF
 
@@ -100,3 +102,5 @@
                                            self.auth_provider, **params)
         self.transfer_accept_client = TransferAcceptClient(
                                            self.auth_provider, **params)
+        self.tsigkey_client = TsigkeyClient(
+                                           self.auth_provider, **params)