Don't start task managers passed in to Connection

If someone (such as our friend nodepool) creates and passes in a
TaskManager, it should be assumed that the calling context controls the
lifecycle of that TaskManager. In that case, don't run start() on it.

Depends-On: https://review.openstack.org/612168
Change-Id: I0ac5dc428250158471cb64d5b1601cabbb4deb86

openstack/connection.py

@@ -288,9 +288,14 @@ class Connection(six.with_metaclass(_meta.ConnectionMeta,
                     load_envvars=cloud is not None,
                     **kwargs)
 
-        self.task_manager = task_manager or _task_manager.TaskManager(
-            self.config.full_name)
-        self.task_manager.start()
+        if task_manager:
+            # If a TaskManager was passed in, don't start it, assume it's
+            # under the control of the calling context.
+            self.task_manager = task_manager
+        else:
+            self.task_manager = _task_manager.TaskManager(
+                self.config.full_name)
+            self.task_manager.start()
 
         self._session = None
         self._proxies = {}

releasenotes/notes/no-start-task-manager-56773f3ea5eb3a59.yaml

@@ -0,0 +1,6 @@
+---
+fixes:
+  - |
+    Fixed a regression in the new `TaskManager` code which caused programs that
+    were passing in a `TaskManager` that they had been running `start` on to
+    fail due to a double call.