Default native threading for sch, api and metadata

This patch switches the default concurrency mode to native threading
for the services that gained native threading support in Flamingo:
nova-scheduler, nova-api, and nova-metadata.

The OS_NOVA_DISABLE_EVENTLET_PATCHING env variable still can be used to
explicitly switch the concurrency mode to eventlet by

  OS_NOVA_DISABLE_EVENTLET_PATCHING=false

We also ensure that the cover, docs, py3xx and functional tox targets
are still running with eventlet while py312-threading kept running
with native threading.

Change-Id: I86c7f31f19ca3345218171f0abfa8ddd4f8fc7ea
Signed-off-by: Balazs Gibizer <gibi@redhat.com>

tox.ini

@@ -56,12 +56,14 @@ commands =
   env TEST_OSPROFILER=1 stestr run --combine --no-discover 'nova.tests.unit.test_profiler'
   stestr slowest
 
-[testenv:{unit,py3,py310,py311,py312}]
+[testenv:{unit,py3,py310,py311,py312,py313}]
 setenv =
   {[testenv]setenv}
 # we do not have any greenlet leaks in unit tests so enforce that
 # by making greenlet leaks a failure.
   NOVA_RAISE_ON_GREENLET_LEAK=True
+# run the test with eventlet
+  OS_NOVA_DISABLE_EVENTLET_PATCHING=False
 
 [testenv:py312-threading]
 setenv =
@@ -81,7 +83,7 @@ commands =
   stestr run {posargs} --exclude-list /tmp/exclude.txt
   stestr slowest
 
-[testenv:functional{,-py310,-py311,-py312}]
+[testenv:functional{,-py310,-py311,-py312,-py313}]
 description =
   Run functional tests.
 setenv =
@@ -89,6 +91,8 @@ setenv =
 # we do not have any greenlet leaks in functional tests so enforce that
 # by making greenlet leaks a failure.
   NOVA_RAISE_ON_GREENLET_LEAK=True
+# run the test with eventlet
+  OS_NOVA_DISABLE_EVENTLET_PATCHING=False
 # As nova functional tests import the PlacementFixture from the placement
 # repository these tests are, by default, set up to run with openstack-placement
 # from pypi. In the gate, Zuul will use the installed version of placement (stable
@@ -208,6 +212,8 @@ commands =
 setenv =
   {[testenv]setenv}
   PYTHON=coverage run --source nova --parallel-mode
+# run the test with eventlet
+  OS_NOVA_DISABLE_EVENTLET_PATCHING=False
 extras =
 commands =
   coverage erase
@@ -244,6 +250,9 @@ commands =
   sphinx-build -W --keep-going -b html -j auto doc/source doc/build/html
   # Test the redirects. This must run after the main docs build
   whereto doc/build/html/.htaccess doc/test/redirect-tests.txt
+setenv =
+  OS_NOVA_DISABLE_EVENTLET_PATCHING=False
+
 
 [testenv:pdf-docs]
 description =