etcdserver: Fix v2v3api set to create parent directly if not exists

When a new file is created under an non existent directly,
the v2 API automatically create the parent directly.
This commit aligns the behaviour of v2v3 emulation to comply with the v2
API.

etcdserver/api/v2v3/store.go

@@ -145,8 +145,10 @@ func (s *v2v3Store) Set(
 	applyf := func(stm concurrency.STM) error {
 		parent := path.Dir(nodePath)
 		if !isRoot(parent) && stm.Rev(s.mkPath(parent)+"/") == 0 {
-			ecode = v2error.EcodeKeyNotFound
-			return nil
+			_, err := s.Create(parent+"/", true, "", false, v2store.TTLOptionSet{})
+			if err != nil {
+				return err
+			}
 		}
 
 		key := s.mkPath(nodePath)