Add zones_export_client's methods and tests to Designate tempest plugin

Partially-Implements: blueprint designate-tempest-plugin

Change-Id: Iff03a53842ac4e44ed720163695a35a74b970768
diff --git a/designate_tempest_plugin/clients.py b/designate_tempest_plugin/clients.py
index d042f65..d461a53 100644
--- a/designate_tempest_plugin/clients.py
+++ b/designate_tempest_plugin/clients.py
@@ -22,6 +22,8 @@
     BlacklistsClient
 from designate_tempest_plugin.services.dns.admin.json.quotas_client import \
     QuotasClient
+from designate_tempest_plugin.services.dns.v2.json.zone_exports_client import \
+    ZoneExportsClient
 
 CONF = config.CONF
 
@@ -44,3 +46,5 @@
                                                      **params)
         self.blacklists_client = BlacklistsClient(self.auth_provider, **params)
         self.quotas_client = QuotasClient(self.auth_provider, **params)
+        self.zone_exports_client = ZoneExportsClient(self.auth_provider,
+                                                     **params)