Initialize the key/directory before watching on kv discovery

Before that change, etcd and zookeeper would fail to instantiate
the discovery without the key being already there in the store or
created beforehand and implicitely by a 'swarm join'.

Signed-off-by: Alexandre Beslic <abronan@docker.com>

discovery/kv/kv.go

@@ -112,6 +112,17 @@ func (s *Discovery) Watch(stopCh <-chan struct{}) (<-chan discovery.Entries, <-c
 		// Forever: Create a store watch, watch until we get an error and then try again.
 		// Will only stop if we receive a stopCh request.
 		for {
+			// Create the path to watch if it does not exist yet
+			exists, err := s.store.Exists(s.path)
+			if err != nil {
+				errCh <- err
+			}
+			if !exists {
+				if err := s.store.Put(s.path, []byte(""), &store.WriteOptions{IsDir: true}); err != nil {
+					errCh <- err
+				}
+			}
+
 			// Set up a watch.
 			watchCh, err := s.store.WatchTree(s.path, stopCh)
 			if err != nil {