Add node callback to flow capable listener from ovsdb southbound

Add node callback to flow capable listener from ovsdb southbound

Use Bridge events in the ovsdb southbound to callback into flowCapableListener.
This callback can be used together (or instead of) the mdsal callback. Also,
protect the nodeCache by using a synchronization object.

Patch 2 (code review):
  - Use Constants.OPENFLOW_NODE_PREFIX instead of duplicate internal copy
  - Make code consistent when removing node form local cache (sync access)
  - Remove check for dpid value (not needed)

Patch 3 (Sonar issue):
  - No need to check for null before an instanceof; the instanceof keyword returns
    false when given a null argument.

Change-Id: Id93cd5f17366e499b7edf2db29164d6f6e586eda
Signed-off-by: Flavio Fernandes <ffernand@redhat.com>