Allow services to start with threading

At the service startup nova need to initialize either the eventlet or
the threading backend of oslo.service. So this patch reuses the existing
logic behind OS_NOVA_DISABLE_EVENTLET_PATCHING.

When OS_NOVA_DISABLE_EVENTLET_PATCHING env variable is set to true the
service will select the threading backend otherwise the eventlet
backend.

Also to avoid later monkey patch calls to invalidated the selection if
the threading backend is selected then the monkey_patch code is
poisoned.

This patch also makes sure that oslo.messaging also initialized with the
matching executor backend.

As this is the last step to make nova-scheduler run in threading mode
this patch adds a release notes as well.

Change-Id: I6e2e6a43df78d23580b5e7402352a5036100ab36
Signed-off-by: Balazs Gibizer <gibi@redhat.com>

doc/source/conf.py

@@ -188,11 +188,3 @@ openstackdocs_projects = [
     'watcher',
 ]
 # -- Custom extensions --------------------------------------------------------
-
-# NOTE(mdbooth): (2019-03-20) Sphinx loads policies defined in setup.cfg, which
-# includes the placement policy at nova/api/openstack/placement/policies.py.
-# Loading this imports nova/api/openstack/__init__.py, which imports
-# nova.monkey_patch, which will do eventlet monkey patching to the sphinx
-# process. As well as being unnecessary and a bad idea, this breaks on
-# python3.6 (but not python3.7), so don't do that.
-os.environ['OS_NOVA_DISABLE_EVENTLET_PATCHING'] = '1'