This on the controller node :

==> neutron/server.log <==
2015-05-06 03:16:55.201 13337 ERROR neutron.notifiers.nova [-] Failed to notify nova on events: [{'status': 'completed', 'tag': u'cd182c4c-67ec-4ad3-9630-dfd5275205ef', 'name': 'network-vif-plugged', 'server_uuid': u'6b8fe0da-7d76-4a35-9821-5d58a7f92c4a'}]
2015-05-06 03:16:55.201 13337 TRACE neutron.notifiers.nova Traceback (most recent call last):
2015-05-06 03:16:55.201 13337 TRACE neutron.notifiers.nova   File "/usr/lib64/python2.7/site-packages/neutron/notifiers/nova.py", line 235, in send_events
2015-05-06 03:16:55.201 13337 TRACE neutron.notifiers.nova     batched_events)
2015-05-06 03:16:55.201 13337 TRACE neutron.notifiers.nova   File "/usr/lib64/python2.7/site-packages/novaclient/v1_1/contrib/server_external_events.py", line 39, in create
2015-05-06 03:16:55.201 13337 TRACE neutron.notifiers.nova     return_raw=True)
2015-05-06 03:16:55.201 13337 TRACE neutron.notifiers.nova   File "/usr/lib64/python2.7/site-packages/novaclient/base.py", line 100, in _create
2015-05-06 03:16:55.201 13337 TRACE neutron.notifiers.nova     _resp, body = self.api.client.post(url, body=body)
2015-05-06 03:16:55.201 13337 TRACE neutron.notifiers.nova   File "/usr/lib64/python2.7/site-packages/novaclient/client.py", line 490, in post
2015-05-06 03:16:55.201 13337 TRACE neutron.notifiers.nova     return self._cs_request(url, 'POST', **kwargs)
2015-05-06 03:16:55.201 13337 TRACE neutron.notifiers.nova   File "/usr/lib64/python2.7/site-packages/novaclient/client.py", line 446, in _cs_request
2015-05-06 03:16:55.201 13337 TRACE neutron.notifiers.nova     self.authenticate()
2015-05-06 03:16:55.201 13337 TRACE neutron.notifiers.nova   File "/usr/lib64/python2.7/site-packages/novaclient/client.py", line 586, in authenticate
2015-05-06 03:16:55.201 13337 TRACE neutron.notifiers.nova     auth_url = self._v2_auth(auth_url)
2015-05-06 03:16:55.201 13337 TRACE neutron.notifiers.nova   File "/usr/lib64/python2.7/site-packages/novaclient/client.py", line 677, in _v2_auth
2015-05-06 03:16:55.201 13337 TRACE neutron.notifiers.nova     return self._authenticate(url, body)
2015-05-06 03:16:55.201 13337 TRACE neutron.notifiers.nova   File "/usr/lib64/python2.7/site-packages/novaclient/client.py", line 690, in _authenticate
2015-05-06 03:16:55.201 13337 TRACE neutron.notifiers.nova     **kwargs)
2015-05-06 03:16:55.201 13337 TRACE neutron.notifiers.nova   File "/usr/lib64/python2.7/site-packages/novaclient/client.py", line 439, in _time_request
2015-05-06 03:16:55.201 13337 TRACE neutron.notifiers.nova     resp, body = self.request(url, method, **kwargs)
2015-05-06 03:16:55.201 13337 TRACE neutron.notifiers.nova   File "/usr/lib64/python2.7/site-packages/novaclient/client.py", line 433, in request
2015-05-06 03:16:55.201 13337 TRACE neutron.notifiers.nova     raise exceptions.from_response(resp, body, url, method)
2015-05-06 03:16:55.201 13337 TRACE neutron.notifiers.nova Unauthorized: The request you have made requires authentication. (HTTP 401)
2015-05-06 03:16:55.201 13337 TRACE neutron.notifiers.nova

i've double checked this on neutron/neutron.conf (controller node)
nova_admin_username = nova
nova_admin_tenant_id = ffa72910a8e449c8a4d6c316331d2744 # service
nova_admin_password = xxx

I have this error in keystone.log :
2015-05-06 03:16:55.195 15313 WARNING keystone.token.controllers [-] User 0ebde333fb514e6bbc567482249b868a is unauthorized for tenant ffa72910a8e449c8a4d6c316331d2744 # service

User 0ebde333fb514e6bbc567482249b868a is indeed my 'nova' user. And according to horizon, it has _admin_ role in tenant ffa72910a8e449c8a4d6c316331d2744 = service