Fix python2 AttributeError with strptime

Fix a potential python2 error when calling strptime, the first call to
strptime is not thread safe and might trigger an exception.
A workaround was added by calling strptime before creating a thread but
the caller is itself within a thread, so it may fail.

The workaround is now to explicitly import _strptime as described in the
upstream python issue https://bugs.python.org/issue7980

Note: This commit only applies to branches that support python2 (initial
commit in stable/train)

Story 2008211
Task 40997

Change-Id: I0e7119d5a379f8662c4913d7232ef28535a29b32

octavia/common/base_taskflow.py

@@ -16,6 +16,9 @@
 import concurrent.futures
 import datetime
 
+# work around for https://bugs.python.org/issue7980
+import _strptime  # noqa: F401 pylint: disable=unused-import
+
 from oslo_config import cfg
 from taskflow import engines as tf_engines
 

releasenotes/notes/fix-python2-attributeerror-strptime-89a7350c55ac8818.yaml

@@ -0,0 +1,6 @@
+---
+fixes:
+  - |
+    Fix a potential AttributeError exception at init time in the housekeeping
+    service when using python2 because of an issue with thread safety when
+    calling strptime for the first time.