We have a need to use the eventlet Event class directly in
oslo.service. Currently it is copy-pasted from this project, but
we now have a duplicate bug due to that so let's just expose it so
one copy can be used in both places.
This should be safe as the class implements a stdlib interface so
we can't really alter the API, and it's not really private since
instances of it are returned to users.
Change-Id: If8e7a41f9fe5573a780f9faabdbaf1326631d37a
Related-Change: https://review.openstack.org/558879
Related-Bug: 1800879