Use in-process ZooKeeper clusters for testing

Use curator's TestingCluster to run in-process ZooKeeper instances for
cluster tests. This is similar to our use of the in-process TestingServer
for tests that require just a single ZooKeeper instance. The difference is
that this is a whole in-process cluster.

This is mostly needed for tests that deal with cluster strategies and
failover.
Get rid of ChildProcesses

ChildProcesses was used for spawning subprocesses to run Zookeeper for
tests. We no longer need it since we run Zookeeper in-process.
Merge pull request #281 from spotify/rohan/fix-tests

Use in-process ZooKeeper clusters for testing