Tweak Tool.java to support the sub command use case

Tool.java provides some methods to handle nice --help output.

for the bin/solr zk subcommands, if you call getUsage you get bin/solr mycommand, but for zk commands we need bin/solr zk mycommand, and Tool.java doesn't support that.

We need a getSubcommand on Tool.java that returns null, but for subcommands they can provide a " zk " string, so we get teh bin/solr zk mycommand output!