Provision an optional client configurable ZK port

Expose an optional parameter that defines the port to start the ZK server on.
This might be particularly helpful with testing infrastructure by having the ability to specify what port ZK server should use to initialize instead of the current default assignment or have to manually edit via zoo.cfg.

Note: The new optional ZK port parameter is applicable with `zkRun` when used to run embedded ZooKeeper with Solr.