swift: avoid Connection aborted

This change removes usage of threads with swift driver.

This avoids to get "Connection aborted" because a thread is stuck
and the server side decide to break the connection.

Related-bug: gnocchixyz#509

gnocchi/service.py

@@ -53,7 +53,12 @@ def prepare_service(args=None, conf=None,
          default_config_files=default_config_files,
          version=gnocchi.__version__)
 
-    utils.parallel_map.MAX_WORKERS = conf.parallel_operations
+    if conf.storage.driver == "swift":
+        # NOTE(sileht): Using threads with swiftclient doesn't work
+        # as expected, so disable it
+        utils.parallel_map.MAX_WORKERS = 1
+    else:
+        utils.parallel_map.MAX_WORKERS = conf.parallel_operations
 
     if not log_to_std and (conf.log_dir or conf.log_file):
         outputs = [daiquiri.output.File(filename=conf.log_file,