Add pkg/discovery for nodes discovery

Absorb Swarm's discovery package in order to provide a common node
discovery mechanism to be used by both Swarm and networking code.

Signed-off-by: Arnaud Porterie <arnaud.porterie@docker.com>

discovery/nodes/nodes.go

@@ -0,0 +1,54 @@
+package nodes
+
+import (
+	"fmt"
+	"strings"
+	"time"
+
+	"github.com/docker/docker/pkg/discovery"
+)
+
+// Discovery is exported
+type Discovery struct {
+	entries discovery.Entries
+}
+
+func init() {
+	Init()
+}
+
+// Init is exported
+func Init() {
+	discovery.Register("nodes", &Discovery{})
+}
+
+// Initialize is exported
+func (s *Discovery) Initialize(uris string, _ time.Duration, _ time.Duration) error {
+	for _, input := range strings.Split(uris, ",") {
+		for _, ip := range discovery.Generate(input) {
+			entry, err := discovery.NewEntry(ip)
+			if err != nil {
+				return fmt.Errorf("%s, please check you are using the correct discovery (missing token:// ?)", err.Error())
+			}
+			s.entries = append(s.entries, entry)
+		}
+	}
+
+	return nil
+}
+
+// Watch is exported
+func (s *Discovery) Watch(stopCh <-chan struct{}) (<-chan discovery.Entries, <-chan error) {
+	ch := make(chan discovery.Entries)
+	go func() {
+		defer close(ch)
+		ch <- s.entries
+		<-stopCh
+	}()
+	return ch, nil
+}
+
+// Register is exported
+func (s *Discovery) Register(addr string) error {
+	return discovery.ErrNotImplemented
+}