Secure ZooKeeper should be easy and the default

ZooKeeper security is documented at https://cwiki.apache.org/confluence/display/solr/ZooKeeper+Access+Control but is not trivial to setup, see http://search-lucene.com/m/eHNlqr6EnMrP6O

As we enable more and more security stuff, securing ZK should be easier to do and ideally the default. This is an umbrella for such improvements.

When all of this is in place and working, perhaps even Solr should refuse to start if Auth/Autz plugins are in use and ZK communication is not properly protected, e.g. require bin/solr start --insecure to override.