Delay using threading.local() to fix check job failure

After I221232d50821fe2adb9881f237f06714003ce79d was merged,
the gate-tempest-dsvm-keystone-eventlet-full has been failing
consistently. We added a threading.local() call and use
that for the oslo.db engine facade. We need to ensure that
monkey patching for eventlet is done before we call local().

Closes-Bug: #1550017
Change-Id: I363ef507e78c1c601011861ebb589c10a2bc489e

keystone/common/sql/core.py

@@ -19,7 +19,6 @@
 
 """
 import functools
-import threading
 
 from oslo_config import cfg
 from oslo_db import exception as db_exception
@@ -184,15 +183,25 @@ def cleanup():
     _main_context_manager = None
 
 
-_CONTEXT = threading.local()
+_CONTEXT = None
+
+
+def _get_context():
+    global _CONTEXT
+    if _CONTEXT is None:
+        # NOTE(dims): Delay the `threading.local` import to allow for
+        # eventlet/gevent monkeypatching to happen
+        import threading
+        _CONTEXT = threading.local()
+    return _CONTEXT
 
 
 def session_for_read():
-    return _get_main_context_manager().reader.using(_CONTEXT)
+    return _get_main_context_manager().reader.using(_get_context())
 
 
 def session_for_write():
-    return _get_main_context_manager().writer.using(_CONTEXT)
+    return _get_main_context_manager().writer.using(_get_context())
 
 
 def truncated(f):