Catch zone import id being None in POST response.

the linked issue suggests that it is possible for zone import id to
be None right upon creation. While I was not able to reproduce this,
let's add an explicit catch for it to make such sutuation more obvious
when this happens again.

Change-Id: I1add5caa29a1ebaf789b1d1123b385de3bf6678d
Related-Issue: PRODX-39453
diff --git a/designate_tempest_plugin/services/dns/v2/json/zone_imports_client.py b/designate_tempest_plugin/services/dns/v2/json/zone_imports_client.py
index 236e737..c8c6ec8 100644
--- a/designate_tempest_plugin/services/dns/v2/json/zone_imports_client.py
+++ b/designate_tempest_plugin/services/dns/v2/json/zone_imports_client.py
@@ -37,6 +37,9 @@
 
         # Create Zone should Return a HTTP 202
         self.expected_success(202, resp.status)
+        msg = "Zone Import created without ID, req-id was {}".format(
+            resp.get("x-openstack-request-id"))
+        assert body['id'], msg
 
         if wait_until:
             waiters.wait_for_zone_import_status(self, body['id'], wait_until)