Looks like we can also hit this when going from neutron to nova for sending external events:

http://logs.openstack.org/23/371523/5/check/gate-tempest-dsvm-neutron-full-ubuntu-xenial/773b57f/logs/screen-q-svc.txt.gz#_2016-10-18_11_58_39_790

2016-10-18 11:58:39.790 2470 ERROR neutron.notifiers.nova [-] Failed to notify nova on events: [{'status': 'completed', 'tag': u'33f6ef2e-dcac-4f4b-8da2-747726c24d5c', 'name': 'network-vif-plugged', 'server_uuid': u'bd23ba0c-a73a-48d5-a491-d1ae489667fe'}]
2016-10-18 11:58:39.790 2470 ERROR neutron.notifiers.nova Traceback (most recent call last):
2016-10-18 11:58:39.790 2470 ERROR neutron.notifiers.nova   File "/opt/stack/new/neutron/neutron/notifiers/nova.py", line 260, in send_events
2016-10-18 11:58:39.790 2470 ERROR neutron.notifiers.nova     batched_events)
2016-10-18 11:58:39.790 2470 ERROR neutron.notifiers.nova   File "/usr/local/lib/python2.7/dist-packages/novaclient/v2/contrib/server_external_events.py", line 39, in create
2016-10-18 11:58:39.790 2470 ERROR neutron.notifiers.nova     return_raw=True)
2016-10-18 11:58:39.790 2470 ERROR neutron.notifiers.nova   File "/usr/local/lib/python2.7/dist-packages/novaclient/base.py", line 356, in _create
2016-10-18 11:58:39.790 2470 ERROR neutron.notifiers.nova     resp, body = self.api.client.post(url, body=body)
2016-10-18 11:58:39.790 2470 ERROR neutron.notifiers.nova   File "/usr/local/lib/python2.7/dist-packages/keystoneauth1/adapter.py", line 205, in post
2016-10-18 11:58:39.790 2470 ERROR neutron.notifiers.nova     return self.request(url, 'POST', **kwargs)
2016-10-18 11:58:39.790 2470 ERROR neutron.notifiers.nova   File "/usr/local/lib/python2.7/dist-packages/novaclient/client.py", line 107, in request
2016-10-18 11:58:39.790 2470 ERROR neutron.notifiers.nova     **kwargs)
2016-10-18 11:58:39.790 2470 ERROR neutron.notifiers.nova   File "/usr/local/lib/python2.7/dist-packages/keystoneauth1/adapter.py", line 356, in request
2016-10-18 11:58:39.790 2470 ERROR neutron.notifiers.nova     resp = super(LegacyJsonAdapter, self).request(*args, **kwargs)
2016-10-18 11:58:39.790 2470 ERROR neutron.notifiers.nova   File "/usr/local/lib/python2.7/dist-packages/keystoneauth1/adapter.py", line 124, in request
2016-10-18 11:58:39.790 2470 ERROR neutron.notifiers.nova     return self.session.request(url, method, **kwargs)
2016-10-18 11:58:39.790 2470 ERROR neutron.notifiers.nova   File "/usr/local/lib/python2.7/dist-packages/positional/__init__.py", line 101, in inner
2016-10-18 11:58:39.790 2470 ERROR neutron.notifiers.nova     return wrapped(*args, **kwargs)
2016-10-18 11:58:39.790 2470 ERROR neutron.notifiers.nova   File "/usr/local/lib/python2.7/dist-packages/keystoneauth1/session.py", line 592, in request
2016-10-18 11:58:39.790 2470 ERROR neutron.notifiers.nova     resp = send(**kwargs)
2016-10-18 11:58:39.790 2470 ERROR neutron.notifiers.nova   File "/usr/local/lib/python2.7/dist-packages/keystoneauth1/session.py", line 642, in _send_request
2016-10-18 11:58:39.790 2470 ERROR neutron.notifiers.nova     raise exceptions.ConnectFailure(msg)
2016-10-18 11:58:39.790 2470 ERROR neutron.notifiers.nova ConnectFailure: Unable to establish connection to https://127.0.0.1:8774/v2.1/os-server-external-events: ('Connection aborted.', BadStatusLine("''",))
2016-10-18 11:58:39.790 2470 ERROR neutron.notifiers.nova