Allow zk driver be imported without zookeeper

The zookeeper python module isn't widely available (e.g. not available
on Fedora), so allow the zk servicegroup driver be imported even if the
module isn't found.

This helps allow the generate_sample.sh script generate a sample config
file even without zookeeper installed.

Change-Id: Ic3b3bb75bcd2003150909ebd2c5724fa5093c346
Merge "Allow zk driver be imported without zookeeper"