Minor code optimization in _compute_topic

In cases where a valid host is passed, the if
statement is always executed twice, not once like it should

Change-Id: I6a24bb4f85bfb94d2fdc5d1652ee371f296a10ef
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 "Minor code optimization in _compute_topic"
Merge "Allow zk driver be imported without zookeeper"