Implement custom command for checking connectivity to KDC, via REST API

There needs to be a way, given the details about a KDC to verify that Ambari and (optionally) the nodes in the existing cluster can connect to it.

From the cluster hosts, this test should test that the address and port combinations are reachable.

From the Ambari server, this test should make sure the administrator credentials allow at least read access to the KDC.

As an example of a similar action, see how Oozie does this for DB check pre install.