Work around strptime threading issue

There is an open bug[1] in python strptime when used in multi-threaded
applications. We have seen this occur in the Octavia test jobs[2].
This patch works around the bug by loading strptime early.

[1] https://bugs.python.org/issue7980
[2] https://logs.opendev.org/37/673337/12/check/ \
    octavia-v2-act-stdby-iptables-dsvm-py2-scenario/440c965/controller/logs \
    /screen-o-cw.txt.gz?level=ERROR#_Aug_09_23_54_29_426364

Change-Id: I932ad625595333e97b0ead074ce64a7341af338d

octavia/common/base_taskflow.py

@@ -14,6 +14,7 @@
 #
 
 import concurrent.futures
+import datetime
 
 from oslo_config import cfg
 from taskflow import engines as tf_engines
@@ -30,6 +31,8 @@ class BaseTaskFlowEngine(object):
     """
 
     def __init__(self):
+        # work around for https://bugs.python.org/issue7980
+        datetime.datetime.strptime('2014-06-19 22:47:16', '%Y-%m-%d %H:%M:%S')
         self.executor = concurrent.futures.ThreadPoolExecutor(
             max_workers=CONF.task_flow.max_workers)