Make OVNMechanismDriver.post_fork_initialize callback cancellable

If the callback method ``OVNMechanismDriver.post_fork_initialize``
fails, the callback manager must raise an exception and finish the
process. If that happens in a Neutron worker, the
``oslo_service.service.Services`` class will restart the process.

NOTE: update n-lib version
The neutron-lib version is bumped to 3.4.2. It contains [1], needed
for this patch.

[1]https://review.opendev.org/c/openstack/neutron-lib/+/903335

Conflicts:
    requirements.txt

Partial-Bug: #2036607
Change-Id: I2aca9a522bda2d69962369748b70fa9270fbe245
(cherry picked from commit bb1114c)

Author: ralonsoh

neutron/plugins/ml2/drivers/ovn/mech_driver/mech_driver.py

@@ -244,7 +244,8 @@ def subscribe(self):
                            events.BEFORE_SPAWN)
         registry.subscribe(self.post_fork_initialize,
                            resources.PROCESS,
-                           events.AFTER_INIT)
+                           events.AFTER_INIT,
+                           cancellable=True)
         registry.subscribe(self._add_segment_host_mapping_for_segment,
                            resources.SEGMENT,
                            events.AFTER_CREATE)

requirements.txt

@@ -20,7 +20,7 @@ Jinja2>=2.10 # BSD License (3 clause)
 keystonemiddleware>=5.1.0 # Apache-2.0
 netaddr>=0.7.18 # BSD
 netifaces>=0.10.4 # MIT
-neutron-lib>=3.4.0 # Apache-2.0
+neutron-lib>=3.4.2 # Apache-2.0
 python-neutronclient>=7.8.0 # Apache-2.0
 tenacity>=6.0.0 # Apache-2.0
 SQLAlchemy>=1.4.23 # MIT