NoneType logging warning with "Received unexpected event network-vif-unplugged" message

[Matt Riedemann]

http://logs.openstack.org/90/417590/9/check/gate-tempest-dsvm-neutron-full-ubuntu-xenial/c831c5d/logs/screen-n-cpu.txt.gz?level=TRACE#_2017-03-20_17_10_19_178

2017-03-20 17:10:19.178 6155 WARNING nova.compute.manager [req-cc7e154f-3cfa-46bd-8af5-d7b4a0871836 service nova] [instance: dae92389-8afc-4666-b290-788e62749524] Received unexpected event network-vif-unplugged-8ed92137-a81a-4afd-a391-854848cd2916 for instance
Traceback (most recent call last):
  File "/usr/lib/python2.7/logging/__init__.py", line 861, in emit
    msg = self.format(record)
  File "/usr/local/lib/python2.7/dist-packages/oslo_log/handlers.py", line 100, in format
    return logging.StreamHandler.format(self, record)
  File "/usr/lib/python2.7/logging/__init__.py", line 734, in format
    return fmt.format(record)
  File "/usr/local/lib/python2.7/dist-packages/oslo_log/formatters.py", line 273, in format
    for arg in record.args:
TypeError: 'NoneType' object is not iterable
Logged from file processutils.py, line 385

Introduced here:

https://review.openstack.org/#/c/381888/1/nova/compute/manager.py@6641