Adding Cross-Realm support for secure Zookeeper client authentication

There is a use case where one needs to support cross realm authentication for zookeeper cluster. One use case is HBase Replication: HBase supports replicating data to multiple slave clusters, where the later might be running in different realms. With current zookeeper security, the region server of master HBase cluster are not able to query the zookeeper quorum members of the slave cluster. This jira is about adding such Xrealm support.