Implement ZooKeeper driver for ServiceGroup API.

The ZooKeeper driver uses ephemeral nodes in ZooKeeper to keep
track of node liveness in a service group. The Implementation is
based on the evzookeeper library to combine zookeeper
and eventlet.

Part of blueprint zk-service-heartbeat

DocImpact: new driver

Change-Id: Ia20519de2b4964007f8b91ea5d56d1875510d40f
Merge "Implement ZooKeeper driver for ServiceGroup API."