Monkey patch original current_thread _active in processutils

Monkey patch the original current_thread to use the up-to-date _active
global variable. This solution is based on that documented at:
eventlet/eventlet#592

Change-Id: I0a2c1e0d8a8cad99d68100d25e88e0d3a2eb8f5c
Related-Bug: #1863021
(cherry picked from commit 6533958)

oslo_concurrency/processutils.py

@@ -56,6 +56,13 @@
         from eventlet.green import subprocess
 
     from eventlet import tpool
+
+    # Monkey patch the original current_thread to use the up-to-date _active
+    # global variable. See https://bugs.launchpad.net/bugs/1863021 and
+    # https://github.com/eventlet/eventlet/issues/592
+    import __original_module_threading as orig_threading
+    import threading
+    orig_threading.current_thread.__globals__['_active'] = threading._active
 else:
     import subprocess