Timeout during creation of interface in functional tests

It happens from time to time that some functional test is failing due to timeout during creation of the interface in a bridge.
Example of such failure: https://storage.bhs.cloud.ovh.net/v1/AUTH_dcaab5e32b234d56b626f72581e3644c/zuul_opendev_logs_c2e/767922/9/gate/neutron-functional-with-uwsgi/c2e764b/testr_results.html

Stack trace:

ft1.1: neutron.tests.functional.agent.linux.test_linuxbridge_arp_protect.LinuxBridgeARPSpoofTestCase.test_arp_correct_protectiontesttools.testresult.real._StringException: traceback-1: {{{
Traceback (most recent call last):
  File "/home/zuul/src/opendev.org/openstack/neutron/.tox/dsvm-functional/lib/python3.8/site-packages/fixtures/fixture.py", line 197, in setUp
    self._setUp()
  File "/home/zuul/src/opendev.org/openstack/neutron/neutron/tests/common/net_helpers.py", line 908, in _setUp
    self.bridge = self._create_bridge()
  File "/home/zuul/src/opendev.org/openstack/neutron/neutron/tests/common/net_helpers.py", line 932, in _create_bridge
    return common_base.create_resource(
  File "/home/zuul/src/opendev.org/openstack/neutron/neutron/tests/common/base.py", line 47, in create_resource
    return creation_func(name, *args, **kwargs)
  File "/home/zuul/src/opendev.org/openstack/neutron/neutron/agent/linux/bridge_lib.py", line 78, in addbr
    bridge.link.create()
  File "/home/zuul/src/opendev.org/openstack/neutron/neutron/agent/linux/ip_lib.py", line 468, in create
    privileged.create_interface(self.name, self._parent.namespace,
  File "/home/zuul/src/opendev.org/openstack/neutron/.tox/dsvm-functional/lib/python3.8/site-packages/oslo_privsep/priv_context.py", line 247, in _wrap
    return self.channel.remote_call(name, args, kwargs)
  File "/home/zuul/src/opendev.org/openstack/neutron/.tox/dsvm-functional/lib/python3.8/site-packages/oslo_privsep/daemon.py", line 214, in remote_call
    result = self.send_recv((Message.CALL.value, name, args, kwargs))
  File "/home/zuul/src/opendev.org/openstack/neutron/.tox/dsvm-functional/lib/python3.8/site-packages/oslo_privsep/comm.py", line 172, in send_recv
    reply = future.result()
  File "/home/zuul/src/opendev.org/openstack/neutron/.tox/dsvm-functional/lib/python3.8/site-packages/oslo_privsep/comm.py", line 109, in result
    self.condvar.wait()
  File "/usr/lib/python3.8/threading.py", line 302, in wait
    waiter.acquire()
  File "/home/zuul/src/opendev.org/openstack/neutron/.tox/dsvm-functional/lib/python3.8/site-packages/eventlet/semaphore.py", line 120, in acquire
    hubs.get_hub().switch()
  File "/home/zuul/src/opendev.org/openstack/neutron/.tox/dsvm-functional/lib/python3.8/site-packages/eventlet/hubs/hub.py", line 313, in switch
    return self.greenlet.switch()
  File "/home/zuul/src/opendev.org/openstack/neutron/.tox/dsvm-functional/lib/python3.8/site-packages/eventlet/hubs/hub.py", line 365, in run
    self.wait(sleep_time)
  File "/home/zuul/src/opendev.org/openstack/neutron/.tox/dsvm-functional/lib/python3.8/site-packages/eventlet/hubs/poll.py", line 80, in wait
    presult = self.do_poll(seconds)
  File "/home/zuul/src/opendev.org/openstack/neutron/.tox/dsvm-functional/lib/python3.8/site-packages/eventlet/hubs/epolls.py", line 31, in do_poll
    return self.poll.poll(seconds)
  File "/home/zuul/src/opendev.org/openstack/neutron/.tox/dsvm-functional/lib/python3.8/site-packages/fixtures/_fixtures/timeout.py", line 52, in signal_handler
    raise TimeoutException()
fixtures._fixtures.timeout.TimeoutException

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/home/zuul/src/opendev.org/openstack/neutron/.tox/dsvm-functional/lib/python3.8/site-packages/fixtures/fixture.py", line 208, in setUp
    raise SetupError(details)
fixtures.fixture.SetupError: {}
}}}

Traceback (most recent call last):
  File "/home/zuul/src/opendev.org/openstack/neutron/.tox/dsvm-functional/lib/python3.8/site-packages/fixtures/fixture.py", line 197, in setUp
    self._setUp()
  File "/home/zuul/src/opendev.org/openstack/neutron/neutron/tests/common/net_helpers.py", line 908, in _setUp
    self.bridge = self._create_bridge()
  File "/home/zuul/src/opendev.org/openstack/neutron/neutron/tests/common/net_helpers.py", line 932, in _create_bridge
    return common_base.create_resource(
  File "/home/zuul/src/opendev.org/openstack/neutron/neutron/tests/common/base.py", line 47, in create_resource
    return creation_func(name, *args, **kwargs)
  File "/home/zuul/src/opendev.org/openstack/neutron/neutron/agent/linux/bridge_lib.py", line 78, in addbr
    bridge.link.create()
  File "/home/zuul/src/opendev.org/openstack/neutron/neutron/agent/linux/ip_lib.py", line 468, in create
    privileged.create_interface(self.name, self._parent.namespace,
  File "/home/zuul/src/opendev.org/openstack/neutron/.tox/dsvm-functional/lib/python3.8/site-packages/oslo_privsep/priv_context.py", line 247, in _wrap
    return self.channel.remote_call(name, args, kwargs)
  File "/home/zuul/src/opendev.org/openstack/neutron/.tox/dsvm-functional/lib/python3.8/site-packages/oslo_privsep/daemon.py", line 214, in remote_call
    result = self.send_recv((Message.CALL.value, name, args, kwargs))
  File "/home/zuul/src/opendev.org/openstack/neutron/.tox/dsvm-functional/lib/python3.8/site-packages/oslo_privsep/comm.py", line 172, in send_recv
    reply = future.result()
  File "/home/zuul/src/opendev.org/openstack/neutron/.tox/dsvm-functional/lib/python3.8/site-packages/oslo_privsep/comm.py", line 109, in result
    self.condvar.wait()
  File "/usr/lib/python3.8/threading.py", line 302, in wait
    waiter.acquire()
  File "/home/zuul/src/opendev.org/openstack/neutron/.tox/dsvm-functional/lib/python3.8/site-packages/eventlet/semaphore.py", line 120, in acquire
    hubs.get_hub().switch()
  File "/home/zuul/src/opendev.org/openstack/neutron/.tox/dsvm-functional/lib/python3.8/site-packages/eventlet/hubs/hub.py", line 313, in switch
    return self.greenlet.switch()
  File "/home/zuul/src/opendev.org/openstack/neutron/.tox/dsvm-functional/lib/python3.8/site-packages/eventlet/hubs/hub.py", line 365, in run
    self.wait(sleep_time)
  File "/home/zuul/src/opendev.org/openstack/neutron/.tox/dsvm-functional/lib/python3.8/site-packages/eventlet/hubs/poll.py", line 80, in wait
    presult = self.do_poll(seconds)
  File "/home/zuul/src/opendev.org/openstack/neutron/.tox/dsvm-functional/lib/python3.8/site-packages/eventlet/hubs/epolls.py", line 31, in do_poll
    return self.poll.poll(seconds)
  File "/home/zuul/src/opendev.org/openstack/neutron/.tox/dsvm-functional/lib/python3.8/site-packages/fixtures/_fixtures/timeout.py", line 52, in signal_handler
    raise TimeoutException()
fixtures._fixtures.timeout.TimeoutException