Fix eventlet detection

Eventlet may be installed, but not used for example projects
like octavia. Improve autodetection mechanism by trying to import
module and check if it is actually patched.

Closes-Bug: #2039346
Change-Id: I860abe953ce945bb5152c77a7daeb6aa1003512b

oslo_log/log.py

@@ -40,6 +40,7 @@ except ImportError:
     syslog = None
 
 from oslo_config import cfg
+from oslo_utils import eventletutils
 from oslo_utils import importutils
 from oslo_utils import units
 
@@ -272,8 +273,7 @@ def _fix_eventlet_logging():
     """
 
     # If eventlet was not loaded before call to setup assume it's not used.
-    eventlet = sys.modules.get('eventlet')
-    if eventlet:
+    if eventletutils.is_monkey_patched('thread'):
         import eventlet.green.threading
         from oslo_log import pipe_mutex
         logging.threading = eventlet.green.threading