Fix gnocchi client import

Related-Prod: PRODX-2479
Change-Id: I84e9a70c81e37ac23fcfc5871c2b6299ddadddef
diff --git a/heat_tempest_plugin/services/clients.py b/heat_tempest_plugin/services/clients.py
index 7604670..a9bd981 100644
--- a/heat_tempest_plugin/services/clients.py
+++ b/heat_tempest_plugin/services/clients.py
@@ -200,5 +200,10 @@
             'session': self.identity_client.session,
             'adapter_options': adapter_options
         }
-        return gnocchi_client.Client(version=self.GNOCCHI_VERSION,
-                                     **args)
+        try:
+            return gnocchi_client.Client(version=self.GNOCCHI_VERSION,
+                                         **args)
+        except ImportError:
+            # catches ujson vs Python GCC compat problem,
+            # see https://github.com/esnme/ultrajson/issues/346
+            return None