Add zones_import_client's methods and tests to Designate tempest plugin

Partially-Implements: blueprint designate-tempest-plugin

Change-Id: If01461617020f39b4da554b127e7b5e5fd704645
diff --git a/designate_tempest_plugin/clients.py b/designate_tempest_plugin/clients.py
index 79d6cba..805eeda 100644
--- a/designate_tempest_plugin/clients.py
+++ b/designate_tempest_plugin/clients.py
@@ -16,7 +16,8 @@
 
 from designate_tempest_plugin.services.dns.v2.json.zones_client import \
     ZonesClient
-
+from designate_tempest_plugin.services.dns.v2.json.zone_imports_client import \
+    ZoneImportsClient
 
 CONF = config.CONF
 
@@ -35,3 +36,5 @@
         params.update(self.default_params)
 
         self.zones_client = ZonesClient(self.auth_provider, **params)
+        self.zone_imports_client = ZoneImportsClient(self.auth_provider,
+                                                     **params)