The Zookeeper Service Group driver has been removed.
The driver has no known users and is not actively maintained. A warning log
message about the driver's state was added for the Kilo release. Also,
evzookeeper library that the driver depends on is unmaintained and
`incompatible with recent eventlet releases`_.
A future release of Nova will `use the Tooz library to track
service liveliness`_, and Tooz supports Zookeeper.
.. _`incompatible with recent eventlet releases`: https://bugs.launchpad.net/nova/+bug/1443910
.. _`use the Tooz library to track service liveliness`: http://specs.openstack.org/openstack/nova-specs/specs/liberty/approved/service-group-using-tooz.html