QPID-5715: [Java Broker] Prevent sporadic failure of BDB HA REST test testNewMasterElectedWhenVirtualHostIsStopped

* VHN role attribute now mutated after the completion of onMaster/onReplica event
* Made BDBHAReplicaVirtualHost a type (BDB_HA_REPLICA) within the VirtualHost category.  This no-op vhost represents the virtualhost
  when the node is replica (and the mastership is elsewhere within the group).

Work by Andrew MacBean <andymacbean@gmail.com> and me.

git-svn-id: https://svn.apache.org/repos/asf/qpid/trunk@1597066 13f79535-47bb-0310-9956-ffa450edef68