Bug-1517: Introduce topology data-change counter

-registers DataChangeListener on specific topology
-increases counter on every "onDataChanged" invocation

-plugin contains sample initial configuration
-topology name is configurable
-can create only sole instance

-exposes data-change count via restconf:
GET "/restconf/operational/data-change-counter:data-change-counter"

Change-Id: I49665f059fa8db0c60491fef1f9424d0b0d87a6b
Signed-off-by: Milos Fabian <milfabia@cisco.com>
Merge "Bug-1517: Introduce topology data-change counter"