ZOOKEEPER-3643: Testing and documenting secure and unsecure ZK client connections

[symat]

We are working in the ZooKeeper SSL integration in HBase. By default, one can enable ZooKeeper SSL client connections using Java System Properties. However, there are certain use-cases, when we need to connect to two ZooKeeper quorum from the same JVM (e.g. when connecting to two HBase clusters for data synchronization). It is possible, that one of the ZooKeeper quorum use SSL while the other doesn't.

In this case it is not possible to use Java System Properties, as those will be affecting both ZooKeeper client connections. These use-cases require code modifications e.g. in HBase to use custom ZooKeeper client configurations. We need to add unit test in ZooKeeper to verify that it works and also make sense to document this use-case to help other open source projects to start using ZooKeeper SSL.

Also, currently it is not possible for the users of the Java Client API to check if an existing ZooKeeper connection is actually uses SSL or not. In this PR a new isSSL() method was added to the ZooKeeper class.

[asf-ci]

Refer to this link for build results (access rights to CI server needed): https://builds.apache.org/job/PreCommit-ZOOKEEPER-github-pr-build-maven/1708/

[symat]

@eolivelli what do you think the suggested improvement on the Java client API?

@anmolnar PTAL