Integration with Docker Discovery

* 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>

driverapi/driverapi.go

@@ -40,6 +40,12 @@ type Driver interface {
 	// Leave method is invoked when a Sandbox detaches from an endpoint.
 	Leave(nid, eid string) error
 
+	// DiscoverNew is a notification for a new discovery event, Example:a new node joining a cluster
+	DiscoverNew(dType DiscoveryType, data interface{}) error
+
+	// DiscoverDelete is a notification for a discovery delete event, Example:a node leaving a cluster
+	DiscoverDelete(dType DiscoveryType, data interface{}) error
+
 	// Type returns the the type of this driver, the network type this driver manages
 	Type() string
 }
@@ -106,3 +112,17 @@ type DriverCallback interface {
 type Capability struct {
 	DataScope datastore.DataScope
 }
+
+// DiscoveryType represents the type of discovery element the DiscoverNew function is invoked on
+type DiscoveryType int
+
+const (
+	// NodeDiscovery represents Node join/leave events provided by discovery
+	NodeDiscovery = iota + 1
+)
+
+// NodeDiscoveryData represents the structure backing the node discovery data json string
+type NodeDiscoveryData struct {
+	Address string
+	Self    bool
+}