Allow Zookeeper servers in TestingCluster to listen on network interfaces other than localhost

Currently, when we create a TestingCluster, all the TestingServers will be bound to the local interface (127.0.0.1). This becomes a constraint if we want to run a TestingCluster which can be used for testing over a network (say LAN).

In order to address that we can add a hostname to be provided optionally in the InstanceSpec.