operation_log: Fix logout generating AttributeError
operation_log: Fix logout generating AttributeError

When logging out request.user.is_authenticated() throws
AttributeError as request.user is NoneType and does not
contain required method.

This patch checks whether request.user is None and
takes a proper action.

Change-Id: Iaef08139e6f5c4c0e886328a16a7ac442acc1c10
Closes-Bug: #1688540

BiznetGIO/horizon,ChameleonCloud/horizon,yeming233/horizon,ChameleonCloud/horizon,NeCTAR-RC/horizon,NeCTAR-RC/horizon,openstack/horizon,yeming233/horizon,NeCTAR-RC/horizon,BiznetGIO/horizon,noironetworks/horizon,ChameleonCloud/horizon,openstack/horizon,noironetworks/horizon,ChameleonCloud/horizon,BiznetGIO/horizon,openstack/horizon,noironetworks/horizon,noironetworks/horizon,BiznetGIO/horizon,yeming233/horizon,yeming233/horizon,NeCTAR-RC/horizon,openstack/horizon