new_solrcloud doesn't validate zk argument

[matthoffman]

if you happen to be dumb, and replay commands from your command history to create new solrcloud clusters without correctly creating the zk ensemble first, sstk will go ahead and try to start up SolrCloud and won't fail until the very end when it waits for Solr to come up.
It's user error, but it's probably easy to validate the zk argument at the beginning to make sure that instances with the given tag exist.