* integrated hostdiscovery package with the new Docker Discovery
* Integrated hostdiscovery package with libnetwork core
* removed libnetwork_discovery tag
* Introduced driver apis for discovery events
* moved overlay driver to make use of the discovery events
* Using Docker Discovery service.
* Changed integration-tests to make use of the new discovery
Signed-off-by: Madhu Venugopal <madhu@docker.com>