YARN-913 add security.md document, move registry architecture document under site/markdown/registry
YARN-913 delete operation is fully asynchronous when issued in YARN complete operations
YARN-913 user registry init is now async.
YARN-913 Ephemeral option dropped from registry API; still available at the ZK level, so the exception is retained
YARN-913 ongoing work to add security
YARN-913 curator service made subclass of CompositeService; provides override point to build the root ACL
YARN-913 more work on registry security
YARN-913 registry security model, initial draft alongside the code