ISSUE

ISSUE
Opendaylight controller to get node description from OF description
statistics datapath description field

CHANGE
- Switch Mgr to expose a getNodeDescription() method which returns the
  description configured by user if any, otherwise the one learnt from
  the proto plugin
- Web bunldes to make use of the above api (Currently they are ignoring
  the info learnt by the plugin)
- OFStatisticsManager to implement a IStatistics interface for informing
  listeners that the description statistics info has been refreshed
- Removing current logic where OFStatisticsManager was explicitely
  invoking the description property update on InventoryService through
  IOFInventoryService
- Removed logic where InventoryServiceShim queries OFStatisticsManager
  about node description as information is not available at that time
  and IStatisticsListener updates will serve same purpose
- Removing statistics request timeouts from OFStatisticsManager as the
  timeout is compeltely handled by core.internal.StatisticsCollector

Change-Id: I0d2941d3012ca6dc77a386570dbbd8e5c7832a03
Signed-off-by: Alessandro Boch <aboch@cisco.com>