Remove implicit RequestContext decoration

This change removes RequestContext implicit decoration as some
applications have their own session management for the moment or don't
need this feature. Moreover this implicit decoration doesn't help
debugging.

This change doesn't disallow RequestContext decoration but applications
should require it explicitly:

 from oslo_db.sqlalchemy import enginefacade
 enginefacade.transaction_context_provider(oslo_context.RequestContext)

Closes-Bug: #1468707
Change-Id: I143f30c41e788c7aa9887c0e994f49ee55c94651

oslo_db/sqlalchemy/enginefacade.py

@@ -18,7 +18,6 @@ import threading
 import warnings
 
 from oslo_config import cfg
-from oslo_context import context as oslo_context
 
 from oslo_db import exception
 from oslo_db import options
@@ -778,10 +777,6 @@ def transaction_context_provider(klass):
     return klass
 
 
-# apply the context descriptors to oslo.context.RequestContext
-transaction_context_provider(oslo_context.RequestContext)
-
-
 _context_manager = _TransactionContextManager(_is_global_manager=True)
 """default context manager."""
 

oslo_db/tests/sqlalchemy/test_enginefacade.py

@@ -35,6 +35,9 @@ from oslo_db.sqlalchemy import orm
 from oslo_db.sqlalchemy import test_base
 
 
+enginefacade.transaction_context_provider(oslo_context.RequestContext)
+
+
 class SingletonOnName(mock.MagicMock):
     def __init__(self, the_name, **kw):
         super(SingletonOnName, self).__init__(