Implement toString in DataBroker impls
PingPongDataBroker extends from ForwardingObject which forwards toString to the
delegate which is the ConcurrentDOMDataBroker. Unfortunately this hides the fact
that it's actually the PingPongDataBroker and has lead to confusion. Similar with the
BindingDOMDataBrokerAdapter. So I implemented toString to appropriately to refect the
true identity and forwarding nature of the classes.
Change-Id: I58b931590ca41bdb90bad31f6fedfe71466787ea
Signed-off-by: Tom Pantelis <tpanteli@brocade.com>